Hardware, and various recyclables such as bottle caps, keys, etc.
Strong disk magnets
Open a tin can with a safety can opener so there are no sharp edges. (If you only have a regular opener, line the inside of the can's rim with electrical tape.)
Empty, wash, and dry the can.
For facial features, arms, propulsion devices, and communication arrays, look for items around the house and at the hardware store. We used bolts, brackets, hinges, keys, wing nuts, switches, bottle caps, washers, knobs, and more.