Balloon Rockets: you know what would make this 100X the fun? Attach a tiny basket to balloons that is big enough for a lego guy to fit in. Instantly you have BALLOON BLIMPS and you're learning about physics BOOM!
behavior management, build your rainbow!!! I would love to use this in tandem with "stay on green" systems. Every day you stay on green you get a rainbow piece! Prize at discovery time after you fill it.