The portable Duck-Under Kaleidoscope. The original is is at The Exploratorium, a science museum in San Francisco. Instructions for building a (non-movable) Duck-Under Kaleidoscope can be found in one of their Cookbooks.
Ours is made of plexiglass mirrors glued to foam core, plastic sewer pipe, and wood. It's a little unstable, and on the heavy side, so we're looking for someone to help us re-engineer it.