build a tumbler/ball mill for cleaning parts and grinding stuff
I think the sander idea is mechanically complicated since you need to make a good level tray, find a drum that fits the sand paper, dust control (its going to get dust into the motor).
A ball mill requires a decent motor but its going to be sealed and you just need to drive some rollers with a belt and you don't need to build a plate to carry the work piece. And you can get a decent deal for sanders, but a ball mill of decent quality is more expensive for what it is.
You can also make a mod for the ball mill to make a machine that gives you the 'stonewash' finish on gadgets that looks nice and is fashionable.