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.
Bottle Bank via Martha Stewart: transform a used plastic bottle into a cute piggy bank. Could use same idea to made little containers for kids to store their collections too (use bottle with wider opening and don't make slot)