As cub scouts, they (or usually their dad) made a car for speed. There are tons of rules and a strong focus on fair play.
I once repeated that in Boy Scouts with a few key exceptions:
1) cars built for durability under violent circumstances, 2) the only rules were that cars had to be built by scouts alone and could not contain explosives.
But anything else was fair game.
Give each scout a pinewood Derby kit. Make or modify a Derby track so that both ends are elevated. Start a car at each end at the same time. A video camera and projector make for a great slo-mo replay. Give awards for most damage done, most parts flying, most inherent trauma to the hypothetical driver, and best hissy-fit thrown by a poor loser.
Depending on how many contestants you have this could take a while. We had about 25 entries so we held it at our Christmas party which is a 2 hour troop meeting.
Again, plant the idea with your SPL and let them fill in all the blanks.