For the annual Soapbox Derby, we scour the streets (and train tracks) for junk that has potential mobility. Old perambulators, discarded refrigerator doors, ATV frames, and dollys are decorated with bear skulls, spray paint, and carpet scraps, and then we fearlessly plunge downhill on the resulting vehicles, which are most always begun (and completed) the night before the race.
See also: http://www.schemingbehemoth.com/paping/happenings/index.html