An old bicycle wheel makes a good pulley wheel.
An old modem, of any sort, can be repurposed by attaching a single RJ-11 cord to both ends of it; you now have a loop, and the RJ-11 threads through the bicycle wheel.
An RS-232 cable should be thought of as a very strong wire rope with helpful screw connectors.
A transformer makes an excellent counterweight.
There is no piece of old electronic junk that doesn't have some possible usefulness as a part of a kinetic sculpture, whether it works or not. Remember, kids, the connector is the artwork.