I got this awesome lesson from the awesome "Dynamic Art Projects For Children" book. If you don't have this I highly recommend it. My 5th graders loved this project and really took their time on it. Like a lot of time, like 5 weeks worth of time...which is really just 5 or 6- 45 minute classes.
First the students chose a letter from the alphabet and drew it large on paper. Then they added letters of various sizes, the rule being all of them had to touch at least one letter. They could also do numbers. Then they traced their letters with black marker and colored all the different shapes with whatever colors they wanted.
Such rad results, don't you think?