I found this in the garage, from a dog we had years ago– it’s called a boomer ball. It’s hard on the outside– virtually indestructible. It has a twist off cap, where you can open it and fill the inside with sand. The more sand you put in it, the more difficult it is for your dog to push– giving him a better workout.
This toy is fantastic if you have a new puppy. Especially one like this, where there is no limit to the number of things you’ll need to keep her mind stimulated.
You can find these boomer balls at most of your big box pet stores or online.