I never know how to respond properly to the question “What’s up?”.
I seems wrong to answer with what I am doing or how I am.
So how do you answer?
My weight.
Sky.
Usually some variant on “Eh. Not much.”
'Cause I’m easygoing like that. 
I’ve started to notice that I, like, LindyHopper usually say “Not much,” regardless of any significant going’s on.
Typical conversation:
Friend: What’s up?
Amazingrace: Not much. You?
Friend: Not much.
Amazingrace: Actually, I just had a flying saucer crash in my backyard. You’ve got to come see it!
And scene.
Another vote here for “Not much.”
That’s pretty much how my conversations go, except for the UFO part.
Yeah, just as in Amazingrace’s example. I answer “Not much” regardless of what’s going on, or if there is something that’s “up” going on.
Yeah, I say “nuthin’ much,” regardless of how much is going on.
Occasionally, if I’m feeling loquacious, I’ll say “not a whole lot.”
Word
To be serious. I hardly ever get asked ‘what’s up’ unless I am showing signs of distress. My mother asked me the other day when she thought I was looking depressed.
But generally the version used in the sense that ‘what’s up’ is used in the OP, is ‘how are you’ or ‘how’s things’, to which I answer ‘fine’ and then I might be more specific… ‘a bit tired though, didn’t get much sleep last night’.
If asked ‘what’s up’ in the conversational way, I’ll say ‘nothing, just…’ (and then describe what I’m having a problem with)
Almost unfailingly, and regardless of the sex of the person asking the question.
Them: “What’s up?”
Me: “You, baby!”
I’m cool like that though.
When I was at high school, I answered the phone and thought that I knew who the caller was but I wasn’t sure. The conversation went:
Me: Hello, Melanie speaking
Caller: Hello Melanie
Me: Who is it?
Caller: You know?
Me: No, I don’t?
Caller: Yes, you do.
Me: Oh, is it you Brian?
Caller: Maybe.
So, What’s up?
Me: (Extended diatribe on what I had been doing in the last few days - went on for about 10 mins)
Caller: So, What’s down?
Me: What do you mean, What’s down?
Caller: Are your knickers down?
Me: (catching on and shocked, quickly hung up)
So, even now, all these years later, when someone asks me ‘what’s up’, I have to fight the urge to shout: ‘MY KNICKERS!’
I use a variation of this response: Word to your mother, or if I am feeling really happy that day “word to your mother’s uncle”.
I often answer:
“It’s a relative indicator of direction, depending on where your feet are.”
That usually gets a “huh?!”
I usually respond by pointing skyward, a smug look on my face.
But if they go to the trouble to say “wuddup” then I respond with the obligatory “wooooord.”
“My nipples, it’s freezing outside!”
That, ot just “Hey.”
When I think about it, I realize I ALWAYS respond to that with “hey.”
“Not much.” Or, “Same old same old.”
[sub](this is the friggin fourth time I’ve tried to post this. Either my ISP is playing silly-buggers or the SDMB is overloaded)[/sub]
Your answer is imperfect. It registers too highly on my pedantry-meter.
My answer, which is designed to address the percieved question: “what is, at this moment, up”, is perfect in the vast majority of situations.
That answer was: ‘sky’