Performance arts [like music (which was mentioned on another thread recently -- a couple were supporting their touring this way), busking, and spray paint art] would be one area.
Arts and crafts would be another. Some people make earrings and other jewelry, and sell them while touring. There might be other crafts that could work as well. Found-object art has books written about it, and touring seems well suited in some ways to this art form.
Body painting might be another way.
Paintings on canvas, paper, or other surfaces would be another area.
Miniature painting would be another.
Any other ideas?