Make Balloon Rockets to race along a string. Whose can travel furthest?
Paper clips, long thread (not string as needs as little friction as possible), balloons, sellotape.
Fasten the thread across the room (making sure everyone knows it's there so they don't walk into it!)
Working in pairs:
Unbend two paper clips to form hooks
Blow up a balloon as far as you dare (don't tie a knot in it but hold the end so the air doesn't escpe yet)
sellotape the paperclip hooks to the top of the balloon
Hook the ballon onto the thread and let go!