I would reshape the moon into the form of a Cuttlefish and make all the farm animals in the world start praying to the moon-god in perfectly understandable [insert local language here].
Also Pigeons can heal amputees, cancer, old age, or anything really, by pooping on them.
Just before my day was up, I would have all the children in the world stop and announce 'Torchwood style' "we will return in 1,000 years, prepare for us." Then I'd leave a 2001 style monolith in every country, but with Bruce Lee engraved on the side.