popping paint filled balloons on a canvas . put paint tape to cover the place to stencil, put paint filled balloons around canvas, pop with darts (slingshots?), let dry, use stencil to add silhouette - love it! Or do the crayon melting thing!
Craft Stick Catapult - I made 4 of these in 5 minutes and they work great! You need craft sticks, 4 rubber bands, hot glue or tape, and a bottle cap (I cut apart an egg carton because I didn't have any bottle caps).
Design a Catapult (for Informal Learning) - Sprinkle