Before lunch I had a look round the public Roman remains which are quite impressive and you can see them for free in the underpasses around Serdika station. Then the archeological museum which was OK. I managed to spend an hour there, but others may struggle to spend as long.
After a bit more of a walk around the city and lunch I went back to the hotel and visited the geological museum in the park by the hotel. I like rocks but it was pretty dull, obviously designed as an educational museum for kids. It does have some very impressive (man sized) geodes if you are into that sort of thing. And (as it proudly claims) a number of reference samples which are the first of various discovered mineral types.

Anyway forget about lovely Sofia for a while, this is Flyertalk and you want to know more about airports and plane food...

The airport lounge at Sofia is fine. It's a contract lounge with the usual offerings. It is downstairs, head down the escalators as if you are exiting the terminal and turn left 180 degrees at the bottom rather than going through the exit that is directly ahead of you. The airport isn't large