How do you test if this is any fun though without much effort?
I had an idea: I would start with the trampoline: go to kmart, buy a mini trampoline (you can even return it there), duck tape a wiimote to the bottom, and connect it to a sound making program like max/msp. then jump! See if it is fun just to jump around and hear different sounds! bring it in next week, you can demo that in 1 min! That might already be enough (at least for me and 10 year old kids!) to be fun and I am sure you'll come up with an even easier way!