Conveyor Belt
It's been done before, but I think if it's done with some careful planning and good modeling, it would provide a lot of interest. The idea is to control the view so only a small portion of the belt is visible. This could be accomplished via windows in a building, or by large objects located at either end of the belt. It should be relatively simple to make with a belt having boxes or other items attached to it, arranged so that the return trip is hidden either behind objects or under the building. Also consider putting one between a tractor trailer and a freight dock.
