Could also connect a bunch of locks and put all the keys on a key ring. Lots of pictures of latch, lock, etc ideas for montessori type preschool. This lock one could be made with 2 boards, some o-rings, and some locks from Dollar Store.
Wall mounted ball run. This would be great for kids who are too young for the marble runs. Could be used as a music installation if there are movable obstructions in the pipe that make sound when a ball touches them