I really enjoy the idea of using LEDs to make temporary graffiti. One way this is done is by attaching magnets to LED nods and throwing them at metallic buildings such as in this video:
Another way this is done sometimes (not always the best thing to do) is to make LED boxs and hang them up around town. A couple years ago the television company Adult Swim hung these LED boxes up around cities unannounced in order to promote the show Aqua Team Hunger Force:
The bad thing about this is that due to it not being announced, the police thought it was a terrorist attack and bomb squads were brought in to remove the light boxes.