Someone came up with the idea of Pretend to be a Time Traveler Day. Definite potential to mess with people's minds and have fun (or look like a total idiot) :) Great comments if you keep reading down the thread.
You could also pretend to be from the very recent future (a la "7 Days"), maybe even only a few hours.
Wear normal clothes, covered in dust. Stagger up to people, grab them and say, "you need to get out of the city now!" then run off.