Every time I visit a new City, the first thing I do is check out if there is a free walking tour. Walking tours are a great way to get to know an area, in a comfortable and informative environment. The local guides/experts can point out to you not only the history of the City you're in, but also any cultural nuances to be aware of.
The Free Sofia Tour did not disappoint, our guide Stefan was witty and knowledgeable with a true passion for the City. The tour runs twice a day, everyday of the year including Christmas and New Years at 11am and 6pm. No reservation is required, just turn up at the Palace of Justice (you can't miss it, 2 huge Lions either side of the steps).