Simple Rocket
Supplies: drinking straw
string
balloon
masking tape
Thread a string through a straw. Have two children hold the string tight between them. Blow up a balloon and have another child hold the end closed while you tape the side of the balloon to the straw as in the picture above. When you say "GO", have the child let go of the end of the balloon to make it "rocket" along the string.
For even more fun, why not form teams and have races. Or experiment to see what shapes or sizes of balloons will "rocket" farther than others.