diff --git a/experiment/src/after_level.rs b/experiment/src/after_level.rs
index 46ba0311e8..9c1e933ea5 100644
--- a/experiment/src/after_level.rs
+++ b/experiment/src/after_level.rs
@@ -1,12 +1,11 @@
 use abstutil::prettyprint_usize;
-use map_gui::SimpleApp;
 use widgetry::{
-    Btn, EventCtx, GfxCtx, HorizontalAlignment, Key, Line, Outcome, Panel, State, Text, Transition,
+    Btn, EventCtx, GfxCtx, HorizontalAlignment, Key, Line, Outcome, Panel, State, Text,
     VerticalAlignment, Widget,
 };
 
 use crate::levels::Level;
-use crate::session::Session;
+use crate::{App, Transition};
 
 const ZOOM: f64 = 2.0;
 
@@ -20,16 +19,14 @@ pub struct Results {
 impl Results {
     pub fn new(
         ctx: &mut EventCtx,
-        app: &SimpleApp,
+        app: &mut App,
         score: usize,
         level: &Level,
-    ) -> Box<dyn State<SimpleApp>> {
+    ) -> Box<dyn State<App>> {
         ctx.canvas.cam_zoom = ZOOM;
         ctx.canvas.center_on_map_pt(app.map.get_bounds().center());
 
-        // TODO Store in app
-        let mut session = Session::new();
-        session.record_score(level.title, score);
+        app.session.record_score(level.title, score);
 
         let mut txt = Text::new();
         txt.add(Line(format!("Results for {}", level.title)).small_heading());
@@ -41,7 +38,7 @@ impl Results {
         )));
         txt.add(Line(""));
         txt.add(Line("High scores:"));
-        for (idx, score) in session.high_scores[level.title].iter().enumerate() {
+        for (idx, score) in app.session.high_scores[level.title].iter().enumerate() {
             txt.add(Line(format!("{}) {}", idx + 1, prettyprint_usize(*score))));
         }
 
@@ -55,8 +52,8 @@ impl Results {
     }
 }
 
-impl State<SimpleApp> for Results {
-    fn event(&mut self, ctx: &mut EventCtx, _: &mut SimpleApp) -> Transition<SimpleApp> {
+impl State<App> for Results {
+    fn event(&mut self, ctx: &mut EventCtx, _: &mut App) -> Transition {
         ctx.canvas_movement();
 
         match self.panel.event(ctx) {
@@ -72,7 +69,7 @@ impl State<SimpleApp> for Results {
         Transition::Keep
     }
 
-    fn draw(&self, g: &mut GfxCtx, _: &SimpleApp) {
+    fn draw(&self, g: &mut GfxCtx, _: &App) {
         self.panel.draw(g);
     }
 }
diff --git a/experiment/src/before_level.rs b/experiment/src/before_level.rs
index 21203bb2ac..1a60cb9afd 100644
--- a/experiment/src/before_level.rs
+++ b/experiment/src/before_level.rs
@@ -2,17 +2,18 @@ use std::collections::HashSet;
 
 use abstutil::prettyprint_usize;
 use map_gui::load::MapLoader;
-use map_gui::{SimpleApp, ID};
+use map_gui::ID;
 use map_model::BuildingID;
 use widgetry::{
     Btn, Choice, Color, EventCtx, GfxCtx, HorizontalAlignment, Key, Line, Outcome, Panel, State,
-    Text, TextExt, Transition, VerticalAlignment, Widget,
+    Text, TextExt, VerticalAlignment, Widget,
 };
 
 use crate::buildings::{BldgState, Buildings};
 use crate::game::Game;
 use crate::levels::Level;
 use crate::vehicles::Vehicle;
+use crate::{App, Transition};
 
 const ZOOM: f64 = 2.0;
 
@@ -24,7 +25,7 @@ pub struct Picker {
 }
 
 impl Picker {
-    pub fn new(ctx: &mut EventCtx, app: &SimpleApp, level: Level) -> Box<dyn State<SimpleApp>> {
+    pub fn new(ctx: &mut EventCtx, app: &App, level: Level) -> Box<dyn State<App>> {
         MapLoader::new(
             ctx,
             app,
@@ -89,8 +90,8 @@ impl Picker {
     }
 }
 
-impl State<SimpleApp> for Picker {
-    fn event(&mut self, ctx: &mut EventCtx, app: &mut SimpleApp) -> Transition<SimpleApp> {
+impl State<App> for Picker {
+    fn event(&mut self, ctx: &mut EventCtx, app: &mut App) -> Transition {
         ctx.canvas_movement();
 
         if ctx.redo_mouseover() {
@@ -132,7 +133,7 @@ impl State<SimpleApp> for Picker {
         Transition::Keep
     }
 
-    fn draw(&self, g: &mut GfxCtx, app: &SimpleApp) {
+    fn draw(&self, g: &mut GfxCtx, app: &App) {
         self.panel.draw(g);
         g.redraw(&self.bldgs.draw_all);
         for b in &self.current_picks {
diff --git a/experiment/src/buildings.rs b/experiment/src/buildings.rs
index 614785542b..c31e52b675 100644
--- a/experiment/src/buildings.rs
+++ b/experiment/src/buildings.rs
@@ -1,9 +1,10 @@
 use std::collections::{HashMap, HashSet};
 
-use map_gui::SimpleApp;
 use map_model::{BuildingID, BuildingType};
 use widgetry::{Color, Drawable, EventCtx, GeomBatch, Line, Text};
 
+use crate::App;
+
 pub struct Buildings {
     // Every building in the map is here, to simplify lookup logic.
     pub buildings: HashMap<BuildingID, BldgState>,
@@ -23,7 +24,7 @@ pub enum BldgState {
 }
 
 impl Buildings {
-    pub fn new(ctx: &mut EventCtx, app: &SimpleApp, upzones: HashSet<BuildingID>) -> Buildings {
+    pub fn new(ctx: &mut EventCtx, app: &App, upzones: HashSet<BuildingID>) -> Buildings {
         let house_color = app.cs.residential_building;
         let apartment_color = Color::CYAN;
         let store_color = Color::YELLOW;
diff --git a/experiment/src/game.rs b/experiment/src/game.rs
index 1a442b3cdc..acf3ef9ef3 100644
--- a/experiment/src/game.rs
+++ b/experiment/src/game.rs
@@ -3,11 +3,10 @@ use std::collections::HashSet;
 use abstutil::prettyprint_usize;
 use geom::{ArrowCap, Circle, Distance, Duration, PolyLine, Pt2D, Time};
 use map_gui::tools::{ChooseSomething, ColorLegend, SimpleMinimap};
-use map_gui::SimpleApp;
 use map_model::BuildingID;
 use widgetry::{
     Btn, Choice, Color, Drawable, EventCtx, GeomBatch, GfxCtx, HorizontalAlignment, Key, Line,
-    Outcome, Panel, State, Text, TextExt, Transition, UpdateType, VerticalAlignment, Widget,
+    Outcome, Panel, State, Text, TextExt, UpdateType, VerticalAlignment, Widget,
 };
 
 use crate::after_level::Results;
@@ -17,6 +16,7 @@ use crate::levels::Level;
 use crate::meters::{custom_bar, make_bar};
 use crate::movement::Player;
 use crate::vehicles::Vehicle;
+use crate::{App, Transition};
 
 const ACQUIRE_BOOST_RATE: f64 = 0.5;
 const BOOST_SPEED_MULTIPLIER: f64 = 2.0;
@@ -39,11 +39,11 @@ pub struct Game {
 impl Game {
     pub fn new(
         ctx: &mut EventCtx,
-        app: &SimpleApp,
+        app: &App,
         level: Level,
         vehicle: Vehicle,
         upzones: HashSet<BuildingID>,
-    ) -> Box<dyn State<SimpleApp>> {
+    ) -> Box<dyn State<App>> {
         let title_panel = Panel::new(Widget::row(vec![
             Btn::svg_def("system/assets/tools/home.svg").build(ctx, "back", Key::Escape),
             "15 min Santa".draw_text(ctx),
@@ -153,8 +153,8 @@ impl Game {
     }
 }
 
-impl State<SimpleApp> for Game {
-    fn event(&mut self, ctx: &mut EventCtx, app: &mut SimpleApp) -> Transition<SimpleApp> {
+impl State<App> for Game {
+    fn event(&mut self, ctx: &mut EventCtx, app: &mut App) -> Transition {
         if let Some(dt) = ctx.input.nonblocking_is_update_event() {
             self.time += dt;
 
@@ -301,7 +301,7 @@ impl State<SimpleApp> for Game {
         Transition::Keep
     }
 
-    fn draw(&self, g: &mut GfxCtx, app: &SimpleApp) {
+    fn draw(&self, g: &mut GfxCtx, app: &App) {
         self.title_panel.draw(g);
         self.status_panel.draw(g);
         self.time_panel.draw(g);
@@ -364,7 +364,7 @@ struct GameState {
 impl GameState {
     fn new(
         ctx: &mut EventCtx,
-        app: &SimpleApp,
+        app: &App,
         level: Level,
         vehicle: Vehicle,
         bldgs: Buildings,
@@ -386,7 +386,7 @@ impl GameState {
         s
     }
 
-    fn recalc_deliveries(&mut self, ctx: &mut EventCtx, app: &SimpleApp) {
+    fn recalc_deliveries(&mut self, ctx: &mut EventCtx, app: &App) {
         let mut batch = GeomBatch::new();
         for (b, state) in &self.bldgs.buildings {
             if let BldgState::Done = state {
@@ -398,12 +398,7 @@ impl GameState {
     }
 
     // If something changed, return the update to the score
-    fn present_dropped(
-        &mut self,
-        ctx: &mut EventCtx,
-        app: &SimpleApp,
-        id: BuildingID,
-    ) -> Option<usize> {
+    fn present_dropped(&mut self, ctx: &mut EventCtx, app: &App, id: BuildingID) -> Option<usize> {
         if !self.has_energy() {
             return None;
         }
@@ -442,7 +437,7 @@ impl EnergylessArrow {
     fn update(
         &mut self,
         ctx: &mut EventCtx,
-        app: &SimpleApp,
+        app: &App,
         time: Time,
         sleigh: Pt2D,
         all_stores: Vec<BuildingID>,
diff --git a/experiment/src/lib.rs b/experiment/src/lib.rs
index b893180f41..19332ba354 100644
--- a/experiment/src/lib.rs
+++ b/experiment/src/lib.rs
@@ -14,11 +14,19 @@ mod session;
 mod title;
 mod vehicles;
 
+type App = map_gui::SimpleApp<session::Session>;
+type Transition = widgetry::Transition<App>;
+
 pub fn main() {
     widgetry::run(widgetry::Settings::new("experiment"), |ctx| {
         let mut opts = map_gui::options::Options::default();
         opts.color_scheme = map_gui::colors::ColorSchemeChoice::NightMode;
-        let app = map_gui::SimpleApp::new_with_opts(ctx, abstutil::CmdArgs::new(), opts);
+        let app = map_gui::SimpleApp::new_with_opts(
+            ctx,
+            abstutil::CmdArgs::new(),
+            opts,
+            session::Session::new(),
+        );
         let states = vec![title::TitleScreen::new(ctx)];
         (app, states)
     });
diff --git a/experiment/src/movement.rs b/experiment/src/movement.rs
index eeb671475e..d03f29c308 100644
--- a/experiment/src/movement.rs
+++ b/experiment/src/movement.rs
@@ -2,11 +2,12 @@ use std::collections::{HashMap, HashSet};
 
 use abstutil::MultiMap;
 use geom::{Angle, Circle, Distance, Pt2D, Speed};
-use map_gui::{SimpleApp, ID};
+use map_gui::ID;
 use map_model::{BuildingID, Direction, IntersectionID, LaneType, RoadID};
 use widgetry::EventCtx;
 
 use crate::controls::InstantController;
+use crate::App;
 
 pub const ZOOM: f64 = 10.0;
 
@@ -19,7 +20,7 @@ pub struct Player {
 }
 
 impl Player {
-    pub fn new(ctx: &mut EventCtx, app: &SimpleApp, start: IntersectionID) -> Player {
+    pub fn new(ctx: &mut EventCtx, app: &App, start: IntersectionID) -> Player {
         ctx.canvas.cam_zoom = ZOOM;
         let pos = app.map.get_i(start).polygon.center();
         ctx.canvas.center_on_map_pt(pos);
@@ -37,7 +38,7 @@ impl Player {
     pub fn update_with_speed(
         &mut self,
         ctx: &mut EventCtx,
-        app: &SimpleApp,
+        app: &App,
         speed: Speed,
     ) -> Vec<BuildingID> {
         let (dx, dy) = self.controls.displacement(ctx, speed);
@@ -52,7 +53,7 @@ impl Player {
     fn apply_displacement(
         &mut self,
         ctx: &mut EventCtx,
-        app: &SimpleApp,
+        app: &App,
         dx: f64,
         dy: f64,
         recurse: bool,
@@ -144,7 +145,7 @@ impl Player {
     }
 
     /// Is the player currently on a road with a bus or bike lane?
-    pub fn on_good_road(&self, app: &SimpleApp) -> bool {
+    pub fn on_good_road(&self, app: &App) -> bool {
         if let On::Road(r, _) = self.on {
             for (_, _, lt) in app.map.get_r(r).lanes_ltr() {
                 if lt == LaneType::Biking || lt == LaneType::Bus {
@@ -164,7 +165,7 @@ enum On {
 }
 
 impl On {
-    fn get_connections(&self, app: &SimpleApp) -> (HashSet<RoadID>, HashSet<IntersectionID>) {
+    fn get_connections(&self, app: &App) -> (HashSet<RoadID>, HashSet<IntersectionID>) {
         let mut valid_roads = HashSet::new();
         let mut valid_intersections = HashSet::new();
         match self {
@@ -198,7 +199,7 @@ struct BuildingsAlongRoad {
 }
 
 impl BuildingsAlongRoad {
-    fn new(app: &SimpleApp) -> BuildingsAlongRoad {
+    fn new(app: &App) -> BuildingsAlongRoad {
         let mut raw: MultiMap<RoadID, (Distance, BuildingID)> = MultiMap::new();
         for b in app.map.all_buildings() {
             // TODO Happily assuming road and lane length is roughly the same
diff --git a/experiment/src/title.rs b/experiment/src/title.rs
index 8cde0730ef..ba485ec641 100644
--- a/experiment/src/title.rs
+++ b/experiment/src/title.rs
@@ -1,18 +1,17 @@
 use map_gui::tools::{open_browser, PopupMsg};
-use map_gui::SimpleApp;
 use widgetry::{
-    Btn, DrawBaselayer, EventCtx, GfxCtx, Key, Line, Outcome, Panel, State, Text, Transition,
-    Widget,
+    Btn, DrawBaselayer, EventCtx, GfxCtx, Key, Line, Outcome, Panel, State, Text, Widget,
 };
 
 use crate::levels::Level;
+use crate::{App, Transition};
 
 pub struct TitleScreen {
     panel: Panel,
 }
 
 impl TitleScreen {
-    pub fn new(ctx: &mut EventCtx) -> Box<dyn State<SimpleApp>> {
+    pub fn new(ctx: &mut EventCtx) -> Box<dyn State<App>> {
         let levels = Level::all();
 
         Box::new(TitleScreen {
@@ -47,8 +46,8 @@ impl TitleScreen {
     }
 }
 
-impl State<SimpleApp> for TitleScreen {
-    fn event(&mut self, ctx: &mut EventCtx, app: &mut SimpleApp) -> Transition<SimpleApp> {
+impl State<App> for TitleScreen {
+    fn event(&mut self, ctx: &mut EventCtx, app: &mut App) -> Transition {
         match self.panel.event(ctx) {
             Outcome::Clicked(x) => match x.as_ref() {
                 "quit" => {
@@ -102,7 +101,7 @@ impl State<SimpleApp> for TitleScreen {
         DrawBaselayer::Custom
     }
 
-    fn draw(&self, g: &mut GfxCtx, app: &SimpleApp) {
+    fn draw(&self, g: &mut GfxCtx, app: &App) {
         g.clear(app.cs.dialog_bg);
         self.panel.draw(g);
     }
diff --git a/fifteen_min/src/isochrone.rs b/fifteen_min/src/isochrone.rs
index f686aba6c1..4bf6a051f2 100644
--- a/fifteen_min/src/isochrone.rs
+++ b/fifteen_min/src/isochrone.rs
@@ -3,10 +3,11 @@ use std::collections::HashMap;
 use abstutil::MultiMap;
 use geom::{Duration, Polygon};
 use map_gui::tools::{amenity_type, Grid};
-use map_gui::SimpleApp;
 use map_model::{connectivity, BuildingID, Map, Path, PathConstraints, PathRequest};
 use widgetry::{Color, Drawable, EventCtx, GeomBatch};
 
+use crate::App;
+
 /// Represents the area reachable from a single building.
 pub struct Isochrone {
     /// The center of the isochrone
@@ -24,7 +25,7 @@ pub struct Isochrone {
 impl Isochrone {
     pub fn new(
         ctx: &mut EventCtx,
-        app: &SimpleApp,
+        app: &App,
         start: BuildingID,
         constraints: PathConstraints,
     ) -> Isochrone {
@@ -57,10 +58,7 @@ impl Isochrone {
     }
 }
 
-fn draw_isochrone(
-    app: &SimpleApp,
-    time_to_reach_building: &HashMap<BuildingID, Duration>,
-) -> GeomBatch {
+fn draw_isochrone(app: &App, time_to_reach_building: &HashMap<BuildingID, Duration>) -> GeomBatch {
     // To generate the polygons covering areas between 0-5 mins, 5-10 mins, etc, we have to feed
     // in a 2D grid of costs. Use a 100x100 meter resolution.
     let bounds = app.map.get_bounds();
diff --git a/fifteen_min/src/main.rs b/fifteen_min/src/main.rs
index 770bdaf5af..dbbe5b41f9 100644
--- a/fifteen_min/src/main.rs
+++ b/fifteen_min/src/main.rs
@@ -4,9 +4,11 @@ mod viewer;
 #[macro_use]
 extern crate log;
 
+type App = map_gui::SimpleApp<()>;
+
 fn main() {
     widgetry::run(widgetry::Settings::new("15-minute neighborhoods"), |ctx| {
-        let app = map_gui::SimpleApp::new(ctx, abstutil::CmdArgs::new());
+        let app = map_gui::SimpleApp::new(ctx, abstutil::CmdArgs::new(), ());
         let states = vec![viewer::Viewer::random_start(ctx, &app)];
         (app, states)
     });
diff --git a/fifteen_min/src/viewer.rs b/fifteen_min/src/viewer.rs
index afb41c2673..983b9af225 100644
--- a/fifteen_min/src/viewer.rs
+++ b/fifteen_min/src/viewer.rs
@@ -6,7 +6,7 @@
 
 use geom::{Distance, Pt2D};
 use map_gui::tools::{amenity_type, nice_map_name, CityPicker, PopupMsg};
-use map_gui::{Cached, SimpleApp, ID};
+use map_gui::{Cached, ID};
 use map_model::{Building, BuildingID, PathConstraints};
 use widgetry::{
     lctrl, Btn, Checkbox, Color, Drawable, EventCtx, GeomBatch, GfxCtx, HorizontalAlignment, Key,
@@ -14,6 +14,7 @@ use widgetry::{
 };
 
 use crate::isochrone::Isochrone;
+use crate::App;
 
 /// This is the UI state for exploring the isochrone/walkshed from a single building.
 pub struct Viewer {
@@ -26,17 +27,13 @@ pub struct Viewer {
 
 impl Viewer {
     /// Start with a random building
-    pub fn random_start(ctx: &mut EventCtx, app: &SimpleApp) -> Box<dyn State<SimpleApp>> {
+    pub fn random_start(ctx: &mut EventCtx, app: &App) -> Box<dyn State<App>> {
         let bldgs = app.map.all_buildings();
         let start = bldgs[bldgs.len() / 2].id;
         Viewer::new(ctx, app, start)
     }
 
-    pub fn new(
-        ctx: &mut EventCtx,
-        app: &SimpleApp,
-        start: BuildingID,
-    ) -> Box<dyn State<SimpleApp>> {
+    pub fn new(ctx: &mut EventCtx, app: &App, start: BuildingID) -> Box<dyn State<App>> {
         let constraints = PathConstraints::Pedestrian;
         let start = app.map.get_b(start);
         let isochrone = Isochrone::new(ctx, app, start.id, constraints);
@@ -52,8 +49,8 @@ impl Viewer {
     }
 }
 
-impl State<SimpleApp> for Viewer {
-    fn event(&mut self, ctx: &mut EventCtx, app: &mut SimpleApp) -> Transition<SimpleApp> {
+impl State<App> for Viewer {
+    fn event(&mut self, ctx: &mut EventCtx, app: &mut App) -> Transition<App> {
         // Allow panning and zooming
         ctx.canvas_movement();
 
@@ -155,7 +152,7 @@ impl State<SimpleApp> for Viewer {
         Transition::Keep
     }
 
-    fn draw(&self, g: &mut GfxCtx, _: &SimpleApp) {
+    fn draw(&self, g: &mut GfxCtx, _: &App) {
         g.redraw(&self.isochrone.draw);
         g.redraw(&self.highlight_start);
         self.panel.draw(g);
@@ -175,12 +172,7 @@ fn draw_star(ctx: &mut EventCtx, center: Pt2D) -> Drawable {
     )
 }
 
-fn build_panel(
-    ctx: &mut EventCtx,
-    app: &SimpleApp,
-    start: &Building,
-    isochrone: &Isochrone,
-) -> Panel {
+fn build_panel(ctx: &mut EventCtx, app: &App, start: &Building, isochrone: &Isochrone) -> Panel {
     let mut rows = Vec::new();
 
     rows.push(Widget::row(vec![
@@ -236,7 +228,7 @@ struct HoverOnBuilding {
 type HoverKey = (BuildingID, f64);
 
 impl HoverOnBuilding {
-    fn key(ctx: &EventCtx, app: &SimpleApp) -> Option<HoverKey> {
+    fn key(ctx: &EventCtx, app: &App) -> Option<HoverKey> {
         match app.mouseover_unzoomed_buildings(ctx) {
             Some(ID::Building(b)) => {
                 let scale_factor = if ctx.canvas.cam_zoom >= app.opts.min_zoom_for_detail {
@@ -252,7 +244,7 @@ impl HoverOnBuilding {
 
     fn value(
         ctx: &mut EventCtx,
-        app: &SimpleApp,
+        app: &App,
         key: HoverKey,
         isochrone: &Isochrone,
     ) -> HoverOnBuilding {
diff --git a/map_gui/src/simple_app.rs b/map_gui/src/simple_app.rs
index 5afffcbf0b..e203d5fd62 100644
--- a/map_gui/src/simple_app.rs
+++ b/map_gui/src/simple_app.rs
@@ -11,20 +11,27 @@ use crate::render::{DrawOptions, Renderable};
 use crate::{AppLike, ID};
 
 /// Simple app state that just renders a static map, without any dynamic agents on the map.
-pub struct SimpleApp {
+pub struct SimpleApp<T> {
     pub map: Map,
     pub draw_map: DrawMap,
     pub cs: ColorScheme,
     pub opts: Options,
     pub current_selection: Option<ID>,
+    /// Custom per-app state can be stored here
+    pub session: T,
 }
 
-impl SimpleApp {
-    pub fn new(ctx: &mut EventCtx, args: CmdArgs) -> SimpleApp {
-        SimpleApp::new_with_opts(ctx, args, Options::default())
+impl<T> SimpleApp<T> {
+    pub fn new(ctx: &mut EventCtx, args: CmdArgs, session: T) -> SimpleApp<T> {
+        SimpleApp::new_with_opts(ctx, args, Options::default(), session)
     }
 
-    pub fn new_with_opts(ctx: &mut EventCtx, mut args: CmdArgs, mut opts: Options) -> SimpleApp {
+    pub fn new_with_opts(
+        ctx: &mut EventCtx,
+        mut args: CmdArgs,
+        mut opts: Options,
+        session: T,
+    ) -> SimpleApp<T> {
         ctx.loading_screen("load map", |ctx, mut timer| {
             opts.update_from_args(&mut args);
             let map_path = args
@@ -42,6 +49,7 @@ impl SimpleApp {
                 cs,
                 opts,
                 current_selection: None,
+                session,
             }
         })
     }
@@ -189,7 +197,7 @@ impl SimpleApp {
     }
 }
 
-impl AppLike for SimpleApp {
+impl<T> AppLike for SimpleApp<T> {
     #[inline]
     fn map(&self) -> &Map {
         &self.map
@@ -244,7 +252,7 @@ impl AppLike for SimpleApp {
         pt: Pt2D,
         target_cam_zoom: Option<f64>,
         _: Option<ID>,
-    ) -> Box<dyn State<SimpleApp>> {
+    ) -> Box<dyn State<SimpleApp<T>>> {
         Box::new(SimpleWarper {
             warper: Warper::new(ctx, pt, target_cam_zoom),
         })
@@ -264,7 +272,7 @@ impl AppLike for SimpleApp {
     }
 }
 
-impl SharedAppState for SimpleApp {
+impl<T> SharedAppState for SimpleApp<T> {
     fn draw_default(&self, g: &mut GfxCtx) {
         self.draw_with_opts(g, DrawOptions::new());
     }
@@ -282,8 +290,8 @@ struct SimpleWarper {
     warper: Warper,
 }
 
-impl State<SimpleApp> for SimpleWarper {
-    fn event(&mut self, ctx: &mut EventCtx, _: &mut SimpleApp) -> Transition<SimpleApp> {
+impl<T> State<SimpleApp<T>> for SimpleWarper {
+    fn event(&mut self, ctx: &mut EventCtx, _: &mut SimpleApp<T>) -> Transition<SimpleApp<T>> {
         if self.warper.event(ctx) {
             Transition::Keep
         } else {
@@ -291,7 +299,7 @@ impl State<SimpleApp> for SimpleWarper {
         }
     }
 
-    fn draw(&self, _: &mut GfxCtx, _: &SimpleApp) {}
+    fn draw(&self, _: &mut GfxCtx, _: &SimpleApp<T>) {}
 }
 
 /// Store a cached key/value pair, only recalculating when the key changes.
diff --git a/map_gui/src/tools/minimap.rs b/map_gui/src/tools/minimap.rs
index 92ddd3502d..1c65e190e0 100644
--- a/map_gui/src/tools/minimap.rs
+++ b/map_gui/src/tools/minimap.rs
@@ -25,7 +25,7 @@ pub struct SimpleMinimap {
 }
 
 impl SimpleMinimap {
-    pub fn new(ctx: &mut EventCtx, app: &SimpleApp, with_zorder: bool) -> SimpleMinimap {
+    pub fn new<T>(ctx: &mut EventCtx, app: &SimpleApp<T>, with_zorder: bool) -> SimpleMinimap {
         // Initially pick a zoom to fit the smaller of the entire map's width or height in the
         // minimap. Arbitrary and probably pretty weird.
         let bounds = app.map.get_bounds();
@@ -49,7 +49,7 @@ impl SimpleMinimap {
         m
     }
 
-    pub fn recreate_panel(&mut self, ctx: &mut EventCtx, app: &SimpleApp) {
+    pub fn recreate_panel<T>(&mut self, ctx: &mut EventCtx, app: &SimpleApp<T>) {
         if ctx.canvas.cam_zoom < app.opts.min_zoom_for_detail {
             self.panel = Panel::empty(ctx);
             return;
@@ -138,7 +138,7 @@ impl SimpleMinimap {
         (pct_x, pct_y)
     }
 
-    pub fn set_zoom(&mut self, ctx: &mut EventCtx, app: &SimpleApp, zoom_lvl: usize) {
+    pub fn set_zoom<T>(&mut self, ctx: &mut EventCtx, app: &SimpleApp<T>, zoom_lvl: usize) {
         // Make the frame wind up in the same relative position on the minimap
         let (pct_x, pct_y) = self.map_to_minimap_pct(ctx.canvas.center_to_map_pt());
 
@@ -154,7 +154,7 @@ impl SimpleMinimap {
         self.offset_y = map_center.y() * self.zoom - pct_y * inner_rect.height();
     }
 
-    fn recenter(&mut self, ctx: &EventCtx, app: &SimpleApp) {
+    fn recenter<T>(&mut self, ctx: &EventCtx, app: &SimpleApp<T>) {
         // Recenter the minimap on the screen bounds
         let map_center = ctx.canvas.center_to_map_pt();
         let rect = self.panel.rect_of("minimap");
@@ -169,11 +169,11 @@ impl SimpleMinimap {
         self.offset_y = off_y.max(0.0).min(bounds.max_y * self.zoom - rect.height());
     }
 
-    pub fn event(
+    pub fn event<T: 'static>(
         &mut self,
         ctx: &mut EventCtx,
-        app: &mut SimpleApp,
-    ) -> Option<Transition<SimpleApp>> {
+        app: &mut SimpleApp<T>,
+    ) -> Option<Transition<SimpleApp<T>>> {
         let zoomed = ctx.canvas.cam_zoom >= app.opts.min_zoom_for_detail;
         if zoomed != self.zoomed {
             let just_zoomed_in = zoomed && !self.zoomed;
@@ -303,11 +303,16 @@ impl SimpleMinimap {
         None
     }
 
-    pub fn draw(&self, g: &mut GfxCtx, app: &SimpleApp) {
+    pub fn draw<T>(&self, g: &mut GfxCtx, app: &SimpleApp<T>) {
         self.draw_with_extra_layers(g, app, Vec::new());
     }
 
-    pub fn draw_with_extra_layers(&self, g: &mut GfxCtx, app: &SimpleApp, extra: Vec<&Drawable>) {
+    pub fn draw_with_extra_layers<T>(
+        &self,
+        g: &mut GfxCtx,
+        app: &SimpleApp<T>,
+        extra: Vec<&Drawable>,
+    ) {
         self.panel.draw(g);
         if !self.zoomed {
             return;
diff --git a/osm_viewer/src/main.rs b/osm_viewer/src/main.rs
index 7c9ff67bc3..6abdd19fbe 100644
--- a/osm_viewer/src/main.rs
+++ b/osm_viewer/src/main.rs
@@ -2,7 +2,7 @@ mod viewer;
 
 fn main() {
     widgetry::run(widgetry::Settings::new("OpenStreetMap viewer"), |ctx| {
-        let app = map_gui::SimpleApp::new(ctx, abstutil::CmdArgs::new());
+        let app = map_gui::SimpleApp::new(ctx, abstutil::CmdArgs::new(), ());
         let states = vec![viewer::Viewer::new(ctx, &app)];
         (app, states)
     });
diff --git a/osm_viewer/src/viewer.rs b/osm_viewer/src/viewer.rs
index 9f3f5b42f0..e897707c05 100644
--- a/osm_viewer/src/viewer.rs
+++ b/osm_viewer/src/viewer.rs
@@ -15,6 +15,8 @@ use widgetry::{
     VerticalAlignment, Widget,
 };
 
+type App = SimpleApp<()>;
+
 pub struct Viewer {
     top_panel: Panel,
     fixed_object_outline: Option<Drawable>,
@@ -23,7 +25,7 @@ pub struct Viewer {
 }
 
 impl Viewer {
-    pub fn new(ctx: &mut EventCtx, app: &SimpleApp) -> Box<dyn State<SimpleApp>> {
+    pub fn new(ctx: &mut EventCtx, app: &App) -> Box<dyn State<App>> {
         let with_zorder = true;
         let mut viewer = Viewer {
             fixed_object_outline: None,
@@ -40,7 +42,7 @@ impl Viewer {
     fn recalculate_top_panel(
         &mut self,
         ctx: &mut EventCtx,
-        app: &SimpleApp,
+        app: &App,
         biz_search_panel: Option<Widget>,
     ) {
         let top_panel = Panel::new(Widget::col(vec![
@@ -75,7 +77,7 @@ impl Viewer {
         self.top_panel = top_panel;
     }
 
-    fn calculate_tags(&self, ctx: &EventCtx, app: &SimpleApp) -> Widget {
+    fn calculate_tags(&self, ctx: &EventCtx, app: &App) -> Widget {
         let mut col = Vec::new();
         if self.fixed_object_outline.is_some() {
             col.push("Click something else to examine it".draw_text(ctx));
@@ -217,8 +219,8 @@ impl Viewer {
     }
 }
 
-impl State<SimpleApp> for Viewer {
-    fn event(&mut self, ctx: &mut EventCtx, app: &mut SimpleApp) -> Transition<SimpleApp> {
+impl State<App> for Viewer {
+    fn event(&mut self, ctx: &mut EventCtx, app: &mut App) -> Transition<App> {
         ctx.canvas_movement();
         if ctx.redo_mouseover() {
             let old_id = app.current_selection.clone();
@@ -347,7 +349,7 @@ impl State<SimpleApp> for Viewer {
         DrawBaselayer::Custom
     }
 
-    fn draw(&self, g: &mut GfxCtx, app: &SimpleApp) {
+    fn draw(&self, g: &mut GfxCtx, app: &App) {
         if g.canvas.cam_zoom < app.opts.min_zoom_for_detail {
             app.draw_unzoomed(g);
         } else {
@@ -378,7 +380,7 @@ struct BusinessSearch {
 }
 
 impl BusinessSearch {
-    fn new(ctx: &mut EventCtx, app: &SimpleApp) -> BusinessSearch {
+    fn new(ctx: &mut EventCtx, app: &App) -> BusinessSearch {
         let mut counts = Counter::new();
         for b in app.map.all_buildings() {
             for a in &b.amenities {
@@ -400,7 +402,7 @@ impl BusinessSearch {
     }
 
     // Updates the highlighted buildings
-    fn update(&mut self, ctx: &mut EventCtx, app: &SimpleApp) {
+    fn update(&mut self, ctx: &mut EventCtx, app: &App) {
         let mut batch = GeomBatch::new();
         for b in app.map.all_buildings() {
             if b.amenities
@@ -413,12 +415,7 @@ impl BusinessSearch {
         self.highlight = ctx.upload(batch);
     }
 
-    fn hovering_on_amenity(
-        &mut self,
-        ctx: &mut EventCtx,
-        app: &SimpleApp,
-        amenity: Option<String>,
-    ) {
+    fn hovering_on_amenity(&mut self, ctx: &mut EventCtx, app: &App, amenity: Option<String>) {
         if amenity.is_none() {
             self.hovering_on_amenity = None;
             return;
diff --git a/parking_mapper/src/main.rs b/parking_mapper/src/main.rs
index ca79711801..0ab87113db 100644
--- a/parking_mapper/src/main.rs
+++ b/parking_mapper/src/main.rs
@@ -2,7 +2,7 @@ mod mapper;
 
 fn main() {
     widgetry::run(widgetry::Settings::new("OSM parking mapper"), |ctx| {
-        let mut app = map_gui::SimpleApp::new(ctx, abstutil::CmdArgs::new());
+        let mut app = map_gui::SimpleApp::new(ctx, abstutil::CmdArgs::new(), ());
         app.opts.min_zoom_for_detail = 2.0;
         let states = vec![mapper::ParkingMapper::new(ctx, &app)];
         (app, states)
diff --git a/parking_mapper/src/mapper.rs b/parking_mapper/src/mapper.rs
index 558a174e3b..d72a23b228 100644
--- a/parking_mapper/src/mapper.rs
+++ b/parking_mapper/src/mapper.rs
@@ -13,6 +13,8 @@ use widgetry::{
     Line, Menu, Outcome, Panel, State, Text, TextExt, Transition, VerticalAlignment, Widget,
 };
 
+type App = SimpleApp<()>;
+
 pub struct ParkingMapper {
     panel: Panel,
     draw_layer: Drawable,
@@ -41,16 +43,16 @@ pub enum Value {
 }
 
 impl ParkingMapper {
-    pub fn new(ctx: &mut EventCtx, app: &SimpleApp) -> Box<dyn State<SimpleApp>> {
+    pub fn new(ctx: &mut EventCtx, app: &App) -> Box<dyn State<App>> {
         ParkingMapper::make(ctx, app, Show::TODO, BTreeMap::new())
     }
 
     fn make(
         ctx: &mut EventCtx,
-        app: &SimpleApp,
+        app: &App,
         show: Show,
         data: BTreeMap<WayID, Value>,
-    ) -> Box<dyn State<SimpleApp>> {
+    ) -> Box<dyn State<App>> {
         let map = &app.map;
 
         let color = match show {
@@ -189,8 +191,8 @@ impl ParkingMapper {
     }
 }
 
-impl State<SimpleApp> for ParkingMapper {
-    fn event(&mut self, ctx: &mut EventCtx, app: &mut SimpleApp) -> Transition<SimpleApp> {
+impl State<App> for ParkingMapper {
+    fn event(&mut self, ctx: &mut EventCtx, app: &mut App) -> Transition<App> {
         let map = &app.map;
 
         ctx.canvas_movement();
@@ -371,7 +373,7 @@ impl State<SimpleApp> for ParkingMapper {
         Transition::Keep
     }
 
-    fn draw(&self, g: &mut GfxCtx, _: &SimpleApp) {
+    fn draw(&self, g: &mut GfxCtx, _: &App) {
         g.redraw(&self.draw_layer);
         if let Some((_, ref roads)) = self.selected {
             g.redraw(roads);
@@ -391,11 +393,11 @@ struct ChangeWay {
 impl ChangeWay {
     fn new(
         ctx: &mut EventCtx,
-        app: &SimpleApp,
+        app: &App,
         selected: &HashSet<RoadID>,
         show: Show,
         data: BTreeMap<WayID, Value>,
-    ) -> Box<dyn State<SimpleApp>> {
+    ) -> Box<dyn State<App>> {
         let map = &app.map;
         let osm_way_id = map
             .get_r(*selected.iter().next().unwrap())
@@ -453,8 +455,8 @@ impl ChangeWay {
     }
 }
 
-impl State<SimpleApp> for ChangeWay {
-    fn event(&mut self, ctx: &mut EventCtx, app: &mut SimpleApp) -> Transition<SimpleApp> {
+impl State<App> for ChangeWay {
+    fn event(&mut self, ctx: &mut EventCtx, app: &mut App) -> Transition<App> {
         ctx.canvas_movement();
         match self.panel.event(ctx) {
             Outcome::Clicked(x) => match x.as_ref() {
@@ -493,7 +495,7 @@ impl State<SimpleApp> for ChangeWay {
         }
     }
 
-    fn draw(&self, g: &mut GfxCtx, _: &SimpleApp) {
+    fn draw(&self, g: &mut GfxCtx, _: &App) {
         g.redraw(&self.draw);
         self.panel.draw(g);
     }
@@ -597,7 +599,7 @@ fn generate_osmc(
     Ok(())
 }
 
-fn find_divided_highways(app: &SimpleApp) -> HashSet<RoadID> {
+fn find_divided_highways(app: &App) -> HashSet<RoadID> {
     let map = &app.map;
     let mut closest: FindClosest<RoadID> = FindClosest::new(map.get_bounds());
     // TODO Consider not even filtering by oneway. I keep finding mistakes where people split a
@@ -640,7 +642,7 @@ fn find_divided_highways(app: &SimpleApp) -> HashSet<RoadID> {
 }
 
 // TODO Lots of false positives here... why?
-fn find_overlapping_stuff(app: &SimpleApp, timer: &mut Timer) -> Vec<Polygon> {
+fn find_overlapping_stuff(app: &App, timer: &mut Timer) -> Vec<Polygon> {
     let map = &app.map;
     let mut closest: FindClosest<RoadID> = FindClosest::new(map.get_bounds());
     for r in map.all_roads() {