[Greg Marra]

One of the things I did in elementary school involved making balloon blimps that could travel the farthest.

By running a piece of string between two points (one end easily disconnectable), then gluing a straw to the top of an inflated-but-not-tied balloon, then letting go of the balloon, the balloon will shoot across the string to the other side of the room.

If you get really tricky, you can get helium balloons, and add just enough weight so the helium balloon and a regular balloon underneath cancel each other out, so when you inflate the regular balloon and let it go, it shoots across at a fairly level height all on its own.