Well. I normally just ignore people. But I guess if I HAD to respond, i'd say "Nothing much."... But dont say that to any sponges, they'd probably just give you a whole list of things to do at the moment. -_-
Some how, questions like this have become a greeting when just saying something like hello would be more apprporiate. An insincere question does not require a sincere response. For me, at work, a good response would be fine, great, ok, good. When I go to happy hour after work, the question becomes more sincere. If I respond with something like "bad", more details are requested. If it hasn't been a bad day I usually keep things positive. Server: How you doing? Me: Not as good as you but I'm OK! Server:How's it going? Me: Fine as frogs hair!