A trampoline sounds like a great idea. Not only is it good exercise, it's fun, too. My daughter would love to have one. We just don't have the space for it.
We have a small trampoline (about 4' across, 8" off the ground) in the schoolroom. My oldest son sometimes gets up in the middle of the night and jumps, when he can't sleep! It cost about $40 at the sporting goods store.
The trampoline takes up the center of my bedroom! I've also considered taking it outside when the weather gets warmer and drier.