Ball blasters!

What we used:

2 toilet paper rolls
tape
2 rubber bands
4 paper clipes
plastic wrap
ping pong ball
scissors

What we did:

Cut one tp tube lengthwise. Tape the cut making it slightly smaller so it will fit inside the other tube. Tape plastic wrap to one end of the smaller tube. Place the smaller tube inside the other tube. Place a 2 paper clips on each tube, across from each other. Attach a rubber band on each side. Place ping pong ball inside tube. Pull smaller tube back and release to launch the ball.

The finished product in action:

This was really fun, once we figured out the paper clips and rubber bands. I kept launching the entire inner tube out. But that worked out with a little switching around.