Wow these are just great! I never thought I'd get such a good response to my thread... Keep em coming guys!
OK what about placing crayons in a "rainbow formation" under the windscreen wiper of every car you can? When they turn them on a rainbow will be drawn across the windscreen!
Or stuff glitter into the air con systems, cue mass of glitter spurting out and glitter sticks around for ages! Better yet have the glitter written on with the name of apostate websites!