Graffiti Research Lab devised a really simple but great looking light graffiti feature, called Throwies. As they describe on their website: "LED Throwies are an inexpensive way to add color to any ferromagnetic surface in your neighborhood. A Throwie consists of a lithium battery, a 10mm diffused LED and a rare-earth magnet taped together. Throw it up high and in quantity to impress your friends and city officials."
When you follow this link, you can see a movie clip of throwies in action in New York City. It looks just lovely. If you feel inspired to try this stuff yourself, check out the instructables website, they have a step-by-step guide on making these throwies.