My go to answer is, "can't complain". Vague enough to leave it alone if the question is trivial, but also open-ended enough to expound upon if the other person is genuinely interested in my well-being.
-- What, "can't complain"? Really? You're in a Siberian prison camp, starving and freezing your toes and fingers off, and you "can't complain"?
-- Yes, when you're in a Siberian prison camp, you can't complain...