This is a cool little craft we found at Disney Family Fun for Teen Tech Week. It takes a little time to prepare for. Just get your friends and family to save some cans, all sizes are great. Using a safe can opener takes away a little work from you, but you can use electric tape to cover up sharp edges. Then use some nuts, bolts, washers, electric wire, duct tape, etc. to make them look all robot-y.
The kids loved it. And if you ask for a handful of hardware from people you know it can be pretty cheap too.