The brachistochrone:

Here are a couple of pics of a brachistochrone demonstrator and a portion of its fabrication. As part of the Calculus Project at the SMM, I am hunting for interesting ways to demonstrate calculus-rich phenomena. A wonderful example, with a very cool history, is a set of ramps which proves that the fastest path downhill, from one point to another, is a specific curve (cycloid) and not a straight line.