Advertisements of all sorts. Graphics, text, all sorts any sorts. (Bob’s iguana bits emporium.)
On the sides of cars. On the sides of buses. On the sides of buildings. Up in the trees. Down in the docks. Keep an eye out, you never know what you might find. Check out the trinity market place for some dated stuff, try more obscure areas then Whitefriar Gate.
Take pictures, many pictures. Of the good ones, and the noticeably bad ones. It’s got to be done.