"Not much" "Nothing much" "Nothing" "Same old, same old (meaning everything is the same as usual, nothing changed)" "The usual" ...and if you want to be sarcastic or a pedant, you can say... "The sky" "The ceiling" "A few clouds at the moment" (since the question is 'what's up' and so you give an answer of what's above you, which is the sky or ceiling LOL)

Yes, "not much" is definitely a common answer. A lot of people reply like : "not much, just doing ____(whatever they are doing)"