Fun little spot the difference game, the hat and face differences are by far the hardest as the game goes on and the coloring difference is the easiest, especially for lighter colors. I agree that adding other variations to movement or their wobble could make it more challenging, but you don't want to have to wait for them to move and your time is just going down because some haven't moved yet. Making 1 not move at all might be a good option to add.