Sofia is not the touristy spot. You will not see many tourists in town. It is more the place for people to live and work and buy the things they need. I loved to visit the open market near the Lions bridge to see the fresh veggies and fruits from local production. And there are a lot of people selling honey near the well. The well, you should visit it when you are in Sofia. It has great warm water free for everyone to get (and take home - the locals came with big bottles to carry their load home).
Sofia has some historical sites. You can see them for free. Roman ruins in a still living city. Sofia is really old. And the churches, you can not miss them. They are beautiful, too. And you see beautiful hills sourrounding the city.
And I loved to visit this restaurant. It is near a lake and they bake their own bread and cook dishes from several parts of Bulgaria.
And both times we were there they had a group singing beautiful folk-songs. In english the name of the restaurant is "once upon a time...", and we really had a good time there.