Boards
Links to pics of witty / funny graffiti around your area, or even stuff that's on teh internetz
Of course, the PEAS bridge on the M25 near me is one. Looked like this:
http://farm4.static.flickr.com/3243/2413128858_b1f98bff68.jpg
and became this:
http://farm4.static.flickr.com/3155/3050069702_4a2afeccb0.jpg
Now you! (No Banksy allowed)