LED Spinning Top
Commercially available plastic LED spinning tops consist of one or more LEDs powered from two button cells and activated using a switch actuated by centrifugal force. More elaborate devices, according to the author's research, also include a microcontroller to provide an ever-changing light display. It is entirely unacceptable, on both pedagogical and environmental grounds, that it is almost always impossible to replace the batteries without damaging the top. This gives us motivation enough to build or own version.