well, two things come to mind that can be fun.
Potato Cannons and Roman Candles... if you make a good Potato Cannon out of Piping you could launch a potato 100s of feet using a can of hairspray, if thats too much work, just shoot roman candles at each other, always fun.. till someone loses an eye.
http://www.angelfire.com/wy/potato/ skimmed through it seems to be right.