A competition between the three to build a small vending machine that dispenses some product, even if it is as mundane as paperclips, marbles, jellybeans, electronic components etc. Must vend three selectable items minimum. Does not have to use coins but bonus if it does. Build in two episodes, compete at the end in the second. Lots of opportunity for artsy stuff, lighting, and unique mechanisms and actuators. After the episode put up a vote for who we think won. Could even make it be made from all salvaged parts and materials.
These can be small table top machines, not full size floor models to save build time and costs.
I think this might give viewers a good example of a start to finish implementation of concept to product. Would really just be fun to watch.