Where does "here for the summer" or "hung out on vacation and banged at the end" stand? Those are the best, as far as casual hookups go, IMO. I wouldn't consider them one night stands, because you're actually hanging out a lot, before it happens. Not "dated" either because you were weren't officially exclusive and never expected it to last, but you did get to know each other + the sexual tension buildup over a longer period of time.
I'd also be very jealous of any "I banged my pen pal" story.