It's pretty cool what you can do with stuff you already have laying around. Once again, Lewis showed me a book of robotics that had some pictures of vibrobots. These are small robots that move using vibration. All it takes is a "AA" battery, a vibrating motor, some wire and hot glue. We used other junk electronics I had laying to add features to our bots. You could use motors from a pager, cell phone or RC car, but mine came from a pair of defunct Xbox controllers.
Lewis was the traditionalist and went with a vibrating bug. Nate and Corinne went with animals near and dear to their hearts, a dog and cat respectively. Lydia pushed her creativity and came out with a dancing frog. All told this took us the better part of a Saturday morning.
These don't stand up well to abuse, and I've re-glued the precious cat multiple times. I dread the battle that awaits when the battery dies...