Place four tennis balls on the ground. Three of them are touching each other, making a sort of pyramid shape. Player must place the 4th ball onto the top of the other three. Here's the catch: they can't use their hands. They can use anything else, just not their hands.
I am not going to give you any hints on how to accomplish this task. But I strongly suggest you try this yourself before introducing it to your students. If they are unable to solve it, they may ask for help or for a demonstration just to prove it is possible.