How about a "Rolling Ball Sculpture" like the one used in the movie Fracture, with Anthony Hopkins.

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If you can't quite tell, It's a kind of rail system that a marble or steel ball runs along through all kinds of complex passages until eventually gravity brings it to the bottom where, in some of the larger models an electric motor can be used to bring it back to the top. I think that's what the wheel is for on the left.

I was fascinated with them in the movie and would certainly love to build one.

The film was good as well.

Pete.