I did a quick scan and it doesn't look like this has been suggested or done before so I thought I'd throw it out there.
Perhaps a "found" challenge. Participants must create something out of found objects... Go for a walk around town, through the woods, etc., and collect your materials- pennies, shoelaces, sticks, pieces that have fallen off cars, chewed gum, what have you, and create a masterpiece.
Also, I got an email the other day about a one-peice-of-paper challenge that some art museum held. The entries were really neat, and if I weren't so technologically challenged I would add a picture or two of them.