Science -- Weather: Fill the cup with water. Put shaving cream on top for a cloud. Explain that when clouds get really heavy with water, it rains! Then put blue food coloring on top of the cloud, and watch it rain. Air and Weather unit
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.