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!
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.
Craft Stick Catapult - my awesome Cub Scout companion leader used this to teach Leave No Trace: need 7 sticks for outdoor code rules, and signature. Band 5 of the sticks, band 2 of the sticks. You rock, Kimberly P.