Balloon Powered Lego Car - There will be hours of fun when the kids make their own balloon powered LEGO Car! It’s such a simple idea but works a treat and it’s a great holiday project. Balloon Powered LEGO Car Tutorial via ‘The Crafty Mummy’
Balloon Rockets - I've done this in several science camps I've conducted. Kids have fun with it. But, when the balloon deflates it may fall off the straw because it's surface area changes - another good thing to show and explain to kids.