Say "I'm about to have coffee, so that's good :-) "

Alternately, instead of trying to prepare for every possible answer, you can constrain the possible replies significantly by being the one who asks the question in the first place. "How's your day going?" is only ever going to get some variation of "good" or "bad". You only need to respond with "great to hear that", or "sorrt to hear that, hope it improves soon". That's it.