As a kid, my favorite checkout line toy was always the water rocket. You know, that egge shaped thing you'd fill with water, then attach to the little tube, and pump the plunger to force air into it so it'd shoot up crazy high and fast? My sister always liked the Silly Putty, and we both enjoyed the sidewalk chalk and a good, old fashioned squirt gun fight, but the water rocket? That made an impact. It shot up in the air with surprise. And, you could show off how strong you were by putting some muscle into that pressure buildup!
The Air Command rockets use the same principle, except they're constructed from empty plastic drink bottles. There are more than thirty designs, and they show you how to make your own, plus the optimum levels of water and air to make that thing zoom.
The site gets a little science-y, if you're into that, and there are lots of pictures and illustrations if you aren't.