Marble Races with a cheap pool noodle (occupies the kids for hours.) Did this, but instead of cutting the pool noodle, the marble is small enough to fit inside the full noodle to roll down. Then can use the noodle for other things too!
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!