While I take the train to London, I have plenty of crazy thinking time and so I think about what I would like to try out. So when I get home I build little simple projects like a 3 speed automatic transmission.
So this works by centrifugal force, the faster the motor turn the yellow thing (will think of a more technical name) the more it expands and pushes the center axle up. This forces the gear changes. The motor is controlled via a RCX as it was part of a display.