Most sandboxes wait for children on a playground or the occasional zen garden raking. This one combines light, sand, and depth cameras to create a completely malleable topographical map you can alter with a brush of your hand.
Gary Glesener, director of the Modeling and Educational Demonstrations Lab (MEDL) at UCLA, set up an augmented reality sandbox that projects topographical color-coding based on the height of that sand. Make a dent and it'll turn into a lake. Form a lump and it'll grow warmer in color.
The project originally began at UC Davis to create a teaching tool for topography. The project has since been restaged in several other places other than UCLA because it's not that hard to set up. All you need is a Microsoft Kinect, a projector, a computer, and a sandbox filled with sand. That might be an expensive bundle in total, but exempting the sandbox you probably have at least two of those things already.
If you want to set it up, 3D developer blog Doc-Ok runs through the steps in detail and on video. In an hour or two you could have the coolest sandbox on the block.