Don't ever give anything far from the standard "not much". An honest answer is really not expected, asking is just a courtesy. If you do say anything else you'll just end up talking about yourself, and no one wants to hear it.

Some other options anyway:
Re: "What's up?"

• "Keeping busy."
• "Same old, same old."
• "Just relaxing."
• "Working hard."

If you don't want them to ask you ever again:

• "I was just headed to the crapper to take a huge dump, my rectum is about to burst and I've already got one poking out... Would you happen to have an extra pair of pants on you?"