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.
Teach with Trash: Tissue Box Science It would be a good idea to use these as mystery boxes. Place six items in boxes and have them guess what the objects are or what they may all have in common. Could be used to introduce a new unit or book.