bristlebot before, you are missing out! This creative summer STEM activity uses a toothbrush, watch battery and pager motor (available on Amazon for about $5).
Kids will probably need help with this one as soldering is required, but it is well worth the effort. By building a bristlebot, kids can learn about electricity, motion or even robots.
Take a look at eGFI (Engineering Go For It) for instruction on how to build the bristlebot.