The San Fransisco Zoo has a fun participatory ad campaign called Critter Quest. They use a series of 4 bus shelter poster depicting pieces of animals. Passersby are encouraged to take a picture of themselves as a peacock, giraffe, butterfly or kudu and send it to the San Fransisco Zoo website gallery. This is a great advertising idea. It's fun, it gets people interested and involved, and it gives people a reason to visit the website. (Found through Marketing Alternatif).