I hit a new GMing milestone the other day. It's not on the actual list, but I'd put it there if I was making one, because it's something that in six-plus years of game mastering I'd never actually managed: I ran a complete, one-session adventure, with a mostly satisfying conclusion. I've run single session deals before, but they all were either irredeemably stupid or ended when people had to go home, rather than having an actual conclusion.
It was a bit humorous, revolving around the rescue of the son of the Mayor of Gnome Town (who was definitely a gnome) but we managed to avoid sending it completely off the rails.
Other items of note: It was almost completely improvised, and everything that wasn't improvised on the spot was less than two hours old. It was one player's first ever game, and although he spent the beginning of it slightly confused, by the conclusion he'd gotten the hang of it and happily wreaking havoc after the Mayor of Gnome Town let slip that the only way to become mayor was to defeat him in single combat.
I had fun. As far as I can tell, the players did, too, and I hope I'm not wrong in that impression. Now I know that with a little enthusiasm and an hour of prep-time I can turn five otherwise normal hours into some quick, dirty, and fun D&D. And that's not a bad thing to be able to do.