What a fantastic idea. The very cool Instructibles website has a recipe for building single-LED ‘throwies’, consisting of a nice bright LED, button cell, and rare earth magnet. The concept is to throw these puppies on to a metal surface as a form of creative grafitti. With an estimated life of a couple of weeks, your local traffic lights or gas station could provide a cool little light show for a few nights at least.
Hell, I’d be keen to hand out a few of these to the local taggers to redirect their efforts. If only there were more biodegradable options or a way to recover the used batteries.