Contra dance. Closely correlated with LessWrong; also correlated with nerdy people in general. I would recommend it to most LessWrongers; it's good even for people who are not generally good at dancing, or who have problems interacting socially. (Perhaps even especially for those people; I think of it as a 'gateway dance.')

Other types of dance, like swing dance. Also some correlation with LessWrong, somewhat recommended but this depends more on your tastes. Generally has a higher barrier to entry than contra dancing.