Sometimes, it's what you don't plan that goes well.
Earth Hour, an event which originated only last year on the other side of the world (Sydney, Australia), and which has gotten some promotion from the World Wildlife Fund, was something which came upon us sustainability administrators almost unheralded.
At Greenback U, it was pushed by a handful of students, it came to our attention late in the game, preparation was interrupted by that annual ritual called Spring Break (no "wildlife" puns, please!), we made only minimal preparations, and it went fantastically well!
Some coordinated darkening of lights on campus, a fair degree of uncoordinated darkening of lights in the residence halls and off campus, some incorporation of activities into a previously scheduled charity event, a lot of information distributed, a lot of enthusiasm received.
Maybe next year, we should do better by not organizing at all?!
(Somehow, I don't think so.)