What to answer to "what's up?"?

Reply to greeting "what's up?"
A: Hey, what's up?
B: Nothing much! (or)
Other examples:-
Nothing much! You tell me? (or)
Nothing much, just reading the newspaper. You tell me? (or)
Nothing much, just finishing up my work. You tell me? (or)
Nothing much, just preparing for an interview. You tell me? or
Etc.