Sofia, Bulgaria – What to do?

In a week I will be flying to Sofia, Bulgaria to speak at a day-long Usability Seminar. I will be spending two extra days (April 15-16) to visit, sightsee, eat, and other activities of travel. Thing is, I have no clue what to do. I welcome suggestions, and if by chance any peterme.com readers live there, I’m happy to meet up!