good response to "What are you up to tonight?"

for context: I'm a college student. Also, there is no Greek life on campus.

Part of me feels like an alpha response would be to respond something like partying, but I don't because 1) usually that's not my plan for the night and 2) I don't really party that much anyways.

A girl asked me this earlier today and my first response (aka on the spot) was to mention rock climbing (we have a rock climbing wall at the college and I was thinking about going later). She responded positively and mentioned that she had gone a few times before too.

but well, obviously "rock climbing" isn't a response that I can repeat. what other responses would be appropriate?