I usually just chuckle a little and say "same ol’ " which is short for “same old shit”.
I think philosophers handle questions like this different than the general population, like “How’s your day” “How are you doing?” “What’s going on?” (to which I usually reply the day of the week)
=)
What’s up? = Same Old
How’s your day? Waking up
How are you doing? Tired
What’s going on? I give the day of the week as my answer.
People actually love these answers for some reason… even people who are clearly not tired, will say, “Yeah… I am too”
I usually say: not a whole lot, what’s up with you?
And then they have the nerve to actually give me a substantive answer. As if I give a shit.
‘What’s up?’
‘hard dicks, helicopters and the price of gas downtown.’
There’s no “should” involved in this. But I usually respond to this - you know - just trying to raise awareness within them 
- by saying: Oh, the sun, the moon, the stars, the clouds, the flying planes, the birds, even you and me, et cetera.
Or I might say: How much time do you have? Let’s get a cup of coffee. But there I’m usually kidding around.
There are all kinds of creative ways in which we could respond to that question.
As long as you don’t say “The sky” or “The ceiling”. Only nerds who aren’t cool enough to hang out with other nerds say that, and it makes me want to punch a kitten every time I hear it.
When people say “What’s up?” to me, I usually reply “Oh, you know…” if I’m in a good mood, or “Same shit, different day” if I’m not.
Unemployment
Insecurity
Taxes
Death rate
I always say, “chillin” when someone asks what’s up.
We just ask a person ‘how’s things’ in the UK 