Here is a silly copy of the Placid Plastic Duck Simulator. The point in the game is... a duck gets dropped into the pool. It will likely gain a bit of movement, enough to carry it to the side of the pool. Where it will bump the side, and get knocked the opposite way. This will continue indefinitely. But here is the exciting bit. Every now and again, a new duck will drop into the pool - with the original one! So you could end up with up to 7 (yes 7) ducks at once!
In terms of future additions there will be more ducks, and different ducks will have different special features. In the original game there is a submarine one, that sometimes dives to the bottom, or one that sprays fire out it's beak. There will be at least one separate area, additional pools that they each could end up in. In the original game, the ducks bump into each other, I'll make some way that they sometimes bump into each other.