Here's a project that involves a different kind of sandbox than the ones you usually think of. This one has a moving wall inside, acting like a piston, to compress the sand. You can make layers using two different colors of sand, and then see what happens when you compress the layers with the piston. If you're handy with woodworking tools, this is a good project to give you a feel for the effects of geological forces that deform the Earth's crust.