A very very common answer, especially if you're only passing by someone, is "nothing much. You?" You can also say this like "nuttin, you?" This means the same as "Nothing. How about you?", but it's shortened to accommodate a passing greeting. If the person has stopped and is actually stopping to hear the answer, the question can be roughly translated as "Hey. What are you up to (now)? or What have you been doing lately?" In this case, you can just tell them what you have been doing since they last saw you. Ex: Studying or just hanging out, etc.

What's up? Nothing much!