I'd like to do a hoax. People seem to believe them so easily.
My brother and perpetrated a hoax at our university back in the 1980's, creating a fake religious cult called The Zenyogi Society. We posted a few signs regarding meetings of Zenyogi Society, sent letters using various pseudonyms to the school newspaper complaining of unsavory activities of the cult.
That was all that was needed....soon, well-intentioned people had letters printed in the newspaper defending the non-existant Zenyogi Society, and then the Dean of Student Affairs began an Official Investigation of the cult. The student newspaper published a very nice editorial explaining that the dean had demanded copies of the Zenyogi letters but that the newspaper had refused to turn them over, citing Freedom of the Press.
It was good fun and I recommend starting a hoax if you ever get the chance, so long as you don't hurt anyone.