Bottle Rockets
I've loved bottle rockets since I was a kid. They are cheap and fun. I soon found that shooting them off individually was boring. I put two in a bottle, twisted the fuses together and lit them both - double the fun. Then I tried 3, 4, and more, with the goal of launching them all at once. This led to my brother, my son and my nephew building machines to launch a bunch of bottle rockets at once. Actually, they are launched serially, but automatically. This year (2004) my nephew built a cool machine (see pictures). There's also a short video here.
The launcher after 1 test flight and after 10 test flights!
2005 UPDATE: A new launcher was built, using thin aluminum as the material of construction. This avoided the PVC melting noted above. Also, more tubes were used on a larger wheel, allowing a TWO GROSS LAUNCH! Check out the video. The final clip shows my nephew and his lovely wife.