I usually fall back on "Just hanging out with friends. Quiet, really." which is very often the truth, but does nicely even when it isn't.

Of course, I also take delight in occasionally saying what I really think, which has me pretty much pegged as the staff whacko, which is also fine.

I don't discuss my relationship life with general co-workers. Or anyone else who's not a close friend. Ain't nobody's business.