I am living in Sofia for 2 years now so I sure can give you a few tips. Depending a bit on what exactly you are aiming at, but you cannot miss Alexander Nevski cathedral and the Russian Church nearby.
There are also a couple of picturesque markets in the city, the famous Boyana church on the foot of Mt. Vitosha and some other landmarks. Unfortunately most of the city is communist-style appartment blocks and their newer cousins, but if you are up for getting a documentary then there’s plenty to do as well.
Drop me an email if you need more information, I might be able to help you out.