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.
Yarn Ball Mobile - Make a solution of glue and water. Wet the yarn with mixture and wrap around an inflated balloon, let dry overnight. pop balloon and you have this great hanging art for the kids room