Logic gates are an abstract device which can be implemented with electromagnetic relays, vacuum tubes, or transistors. These implemenations have been successful in computing in part because of various properties of chainability, durability, and size beyond their basic binary stability. They also work well because electricity is the energy source which can rather easily be shipped around.
I've seen adders built out of wood, marbles, and gravity. I've seen "lab on a chip" capilary-action-driven prototypes. I've seen all kinds of specialty mechanical calculators (Curta, slide rule). I've seen domino trails as single-use logic gates.
I'm interested in other illustrative computing devices that aren't necessarily convenient, durable, or fast, but which exploit properties of everyday materials to perform computation and which are directly visible. The dominoes trails are close, but are a little too complicated to reset.
Magneto-mechanical arrangements? Water in pipes/troughs? More general marble contraptions?
PS. Here's a new one. Mechanical CPU Clock