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!
This is a great idea for stamping and color mixing for younger kids still working on their fine motor skills (as long as there's close supervision with those balloons, of course!) Painting with balloons