diff --git a/js/game/model/CountingGameLevel.ts b/js/game/model/CountingGameLevel.ts
index f662c01d..b4bc1552 100644
--- a/js/game/model/CountingGameLevel.ts
+++ b/js/game/model/CountingGameLevel.ts
@@ -17,6 +17,8 @@ import PlayObjectType from '../../../../counting-common/js/common/model/PlayObje
 import dotRandom from '../../../../dot/js/dotRandom.js';
 import BooleanProperty from '../../../../axon/js/BooleanProperty.js';
 import numberPlayStrings from '../../numberPlayStrings.js';
+import IReadOnlyProperty from '../../../../axon/js/IReadOnlyProperty.js';
+import Enumeration from '../../../../phet-core/js/Enumeration.js';
 
 // constants
 const LEVEL_INPUT_RANGE = 10;
@@ -24,8 +26,10 @@ const LEVEL_INPUT_RANGE = 10;
 class CountingGameLevel extends NumberPlayGameLevel {
 
   public readonly objectsPlayArea: OnesPlayArea;
-  public readonly playObjectTypeProperty: EnumerationProperty;
-  public readonly isObjectsRepresentationProperty: BooleanProperty;
+  private readonly _playObjectTypeProperty: EnumerationProperty;
+  public readonly playObjectTypeProperty: IReadOnlyProperty<Enumeration>;
+  private readonly _isObjectsRepresentationProperty: BooleanProperty;
+  public readonly isObjectsRepresentationProperty: IReadOnlyProperty<boolean>;
   public readonly groupObjects: boolean;
 
   constructor( levelNumber: number ) {
@@ -42,11 +46,14 @@ class CountingGameLevel extends NumberPlayGameLevel {
     } );
 
     // the object type of the current challenge
-    this.playObjectTypeProperty = new EnumerationProperty( PlayObjectType, CountingGameLevel.getRandomPlayObjectType() );
+    // TODO-TS: Use updated enumeration pattern for Property when PlayObjectType is converted. See https://github.com/phetsims/number-play/issues/80
+    this._playObjectTypeProperty = new EnumerationProperty( PlayObjectType, CountingGameLevel.getRandomPlayObjectType() );
+    this.playObjectTypeProperty = this._playObjectTypeProperty;
 
     // whether the current representation of the challengeNumber are objects. Always use objects as the first representation
     // of the current challenge
-    this.isObjectsRepresentationProperty = new BooleanProperty( true );
+    this._isObjectsRepresentationProperty = new BooleanProperty( true );
+    this.isObjectsRepresentationProperty = this._isObjectsRepresentationProperty;
   }
 
   /**
@@ -60,8 +67,8 @@ class CountingGameLevel extends NumberPlayGameLevel {
 
   public reset(): void {
     super.reset();
-    this.playObjectTypeProperty.reset();
-    this.isObjectsRepresentationProperty.reset();
+    this._playObjectTypeProperty.reset();
+    this._isObjectsRepresentationProperty.reset();
   }
 
   public step( dt: number ): void {
@@ -72,8 +79,8 @@ class CountingGameLevel extends NumberPlayGameLevel {
    */
   public newChallenge(): void {
     super.newChallenge();
-    this.playObjectTypeProperty.value = CountingGameLevel.getRandomPlayObjectType();
-    this.isObjectsRepresentationProperty.value = !this.isObjectsRepresentationProperty.value;
+    this._playObjectTypeProperty.value = CountingGameLevel.getRandomPlayObjectType();
+    this._isObjectsRepresentationProperty.value = !this._isObjectsRepresentationProperty.value;
   }
 }
 
diff --git a/js/game/model/NumberPlayGameLevel.ts b/js/game/model/NumberPlayGameLevel.ts
index 57e8dcb1..301ae8a0 100644
--- a/js/game/model/NumberPlayGameLevel.ts
+++ b/js/game/model/NumberPlayGameLevel.ts
@@ -19,7 +19,7 @@ abstract class NumberPlayGameLevel {
   public readonly scoreProperty: NumberProperty;
   public readonly isChallengeSolvedProperty: BooleanProperty;
   public readonly challengeRange: Range;
-  public readonly challengeNumberProperty: NumberProperty;
+  public readonly challengeNumberProperty: NumberProperty; // TODO-TS: This should be IReadOnlyProperty. See https://github.com/phetsims/number-play/issues/81.
   private oldChallengeNumberOne: number;
   private oldChallengeNumberTwo: number;
   public readonly numberOfAnswerButtonPressesProperty: NumberProperty;
diff --git a/js/game/model/Subitizer.ts b/js/game/model/Subitizer.ts
index b8c6ad61..ad5f95bc 100644
--- a/js/game/model/Subitizer.ts
+++ b/js/game/model/Subitizer.ts
@@ -126,18 +126,21 @@ class Subitizer {
   private readonly challengeNumberProperty: NumberProperty;
   private readonly isChallengeSolvedProperty: BooleanProperty;
   public readonly isShapeVisibleProperty: BooleanProperty;
-  public readonly pointsProperty: Property<Vector2[]>;
+  private readonly _pointsProperty: Property<Vector2[]>;
+  public readonly pointsProperty: IReadOnlyProperty<Vector2[]>;
   private readonly randomOrPredetermined: boolean;
   private timeSinceShapeVisible: number;
   public readonly objectSize: number;
   public readonly isInputEnabledProperty: BooleanProperty;
   private timeToShowShapeProperty: IReadOnlyProperty<number>;
-  public readonly objectTypeProperty: Property<SubitizeObjectTypeEnum>;
+  private readonly _objectTypeProperty: Property<SubitizeObjectTypeEnum>;
+  public readonly objectTypeProperty: IReadOnlyProperty<SubitizeObjectTypeEnum>;
   private isDelayStarted: boolean;
   private timeSinceDelayStarted: number;
   public readonly isLoadingBarAnimatingProperty: BooleanProperty;
   public static SUBITIZER_BOUNDS: Bounds2;
-  public readonly isPlayButtonVisibleProperty: BooleanProperty;
+  private readonly _isPlayButtonVisibleProperty: BooleanProperty;
+  public readonly isPlayButtonVisibleProperty: IReadOnlyProperty<boolean>;
 
   constructor( challengeNumberProperty: NumberProperty,
                isChallengeSolvedProperty: BooleanProperty,
@@ -148,7 +151,8 @@ class Subitizer {
     this.isChallengeSolvedProperty = isChallengeSolvedProperty;
 
     // whether the play button is visible
-    this.isPlayButtonVisibleProperty = new BooleanProperty( true );
+    this._isPlayButtonVisibleProperty = new BooleanProperty( true );
+    this.isPlayButtonVisibleProperty = this._isPlayButtonVisibleProperty;
 
     // whether the loading bar is animating. This can also be used to stop an existing animation.
     this.isLoadingBarAnimatingProperty = new BooleanProperty( false );
@@ -157,10 +161,11 @@ class Subitizer {
     this.isShapeVisibleProperty = new BooleanProperty( false );
 
     // the points of the current shape
-    this.pointsProperty = new Property( [ Vector2.ZERO ], {
+    this._pointsProperty = new Property( [ Vector2.ZERO ], {
       valueType: Array,
       arrayElementType: Vector2
     } );
+    this.pointsProperty = this._pointsProperty;
 
     // if true, make random or predetermined shapes. if false, only make arranged shapes.
     this.randomOrPredetermined = randomOrPredetermined;
@@ -183,7 +188,8 @@ class Subitizer {
     this.isInputEnabledProperty = new BooleanProperty( false );
 
     // the object type of the current shape
-    this.objectTypeProperty = new StringEnumerationProperty( SubitizeObjectTypeValues, 'dog' );
+    this._objectTypeProperty = new StringEnumerationProperty( SubitizeObjectTypeValues, 'dog' );
+    this.objectTypeProperty = this._objectTypeProperty;
 
     // how long the shape is visible when shown, in seconds. This is a derived Property instead of a constant because
     // the time that the shape is shown is increased if the user gets the answer wrong multiple times.
@@ -256,7 +262,7 @@ class Subitizer {
       this.isLoadingBarAnimatingProperty.reset();
       this.resetDelay();
       this.resetShapeVisible();
-      this.isPlayButtonVisibleProperty.reset();
+      this._isPlayButtonVisibleProperty.reset();
       this.isInputEnabledProperty.reset();
     }
   }
@@ -298,8 +304,8 @@ class Subitizer {
                                                          `${challengeNumber}: ${points.length}` );
 
     // two of the same shapes in a row are not allowed
-    if ( !Subitizer.arePointsEqual( this.pointsProperty.value, points ) ) {
-      this.pointsProperty.value = points;
+    if ( !Subitizer.arePointsEqual( this._pointsProperty.value, points ) ) {
+      this._pointsProperty.value = points;
     }
     else {
       this.setNewPoints();
@@ -310,13 +316,13 @@ class Subitizer {
    * Sets this.objectTypeProperty with a new object type for the current challenge.
    */
   private setRandomPlayObjectType(): void {
-    this.objectTypeProperty.value = dotRandom.sample( SubitizeObjectTypeValues.slice() );
+    this._objectTypeProperty.value = dotRandom.sample( SubitizeObjectTypeValues.slice() );
   }
 
   public reset(): void {
     this.isShapeVisibleProperty.reset();
     this.isInputEnabledProperty.reset();
-    this.isPlayButtonVisibleProperty.reset();
+    this._isPlayButtonVisibleProperty.reset();
   }
 
   /**