I'm taking the May 21 Bulgaria tour and I have a really good flight option (with miles) but it gets me to Sofia with two full days before the tour. Is there enough to see in Sofia to warrant arriving that early?
I loved this tour! I spent an extra day in Sofia both at the beginning and end of the tour. I had no trouble filling the time. I used the Lonely Planet guidebook to get ideas
If you want something more “formal”, you could look at the day tours offered by Lyuba Tours:
This company is run by a couple of Rick Steves tour guides - you’ll probably have one of them on your tour! They are excellent.