This is this years Pinewood Derby entry. I wanted to have lights. I got em'. But I wanted to be able to change the settings on the fly, which is not too easy as the car would be on the track.
I decided to use both the Arduino and XBee wireless radio to get'r done. The Arduino I am using, is the Funnle IO, or FIO. It's very small, has an on board LiPo charger and Xbee socket. Perfect for this situation. I designed and etched a custom PCBs for the connectivity to the FIO as well as the LED board. All this had to fit into the pinewood derby car and stay under 5 ounces. It was awesome watching the kids react to it changing as I sat on the other side of the room sending it via my laptop.
Edit: Clarification - I am not employing a "joule thief" circuit in the car. We get to name the cars, and I felt that the name fit.