I've been playing around with mechanical things you can make from cardboard, and I made this cardboard spring. It's really simple, and fun to sproing, hop, or just wave around!
Cut out some cardboard squares.
Peel back the cardboard.
Tape peeled edges together.
P.S. If you think of something cool to make using this (and it's still before 7/13/12), you should enter it in my Cardboard Challenge!