Any ideas for a desk ornament with moving parts...?

How young is he? In the 70s there was a popular "executive desk toy" that was a frame with several small steel suspended from wires. The idea was that you'd take the one on the end, pull it out a ways, and let it fall back to hit the others. The one on the other end would then fly out and fall back, causing the first one to reciprocate. It was supposed to be a perpetual motion machine, I think, as it would keep going forever on its own unless you stopped it. The well-made ones did, anyway; they were properly balanced. If your friend is young enough that he wouldn't know about those, I'd suggest that. To someone who wasn't around at that time, it would be a new idea.