These light graffiti cans give the feeling that you're spray painting with light. Combine these with the free software at http://glowdoodle.com/
and you've got a virtual 3D graffiti wall. You can also use these with a long camera exposure in a dark room.
This version is very easy to build and requires few tools. For building these with real spray paint cans, check out this tutorial: http://www.instructables.com/id/DIY-8-Light-Spray-Graffiti/