If your arrival goes smoothly with no delays I would say there's enough time to go straight into the city for an hour, especially if you only have a carry on. I'm willing to bet coming in J, you can be out of the airport in less than 10 minutes.

Been a few times to UB and honestly don't have any favorites there, more of just going to random places to get food in me. Plus in January it'll be quite cold, around -5F during the day, so being outside for long isn't practical. I would say to get a taxi to the square. You'll be able to snap a few quick pics of the palace and there's a tourist information center along the main road in the same building as the bank (look away from the blue building). They'll be able to give you a better suggestion for food than me.