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.
Come Together Kids: Mini Marshmallow Shooters (or Pom Pom Poppers) put a balloon over the top half of a stiff plastic cup, with the bottom of the cup removed. Pull of the balloon tie with the pompom inside, and POP!
How to Turn Community Cleanup Into a Fun Scavenger Hunt