In my case its to get them used to using hand tools (wrenches, screwdrivers, etc) and to show them mechanical movements. I usually ask them to cover one side of a sheet of paper with something about what they found. (I say I want to see what they found interesting – it could be a drawing of some part they discovered, a haiku about tools, or a description of something you would make using some of the stuff you found inside the thing)

So go shop curb depot (aka, trash night) and grab interesting things. Old windup alarm clocks are great things (and don’t have nasty fluids to drain first). An old VCR while it is electronic, has lots of complicated bits to pull the tape out of the cartridge, and most kids love spinning the head drum (they are heavy, and have good bearing, so inertia is great tactile feel). A printer that feeds sheets of paper is usually good for a geartrain or three, and the DC motors (not all are brushless usually) are great things to play with afterwards. The circuit cards can be cut up with a coping saw, for collage, techno-mosaic, or Christmas ornaments..