Do you have any good travel tips for Sofia (Bulgaria)?

i.e. where to go, what to see etc.

You should definetely visit the Alexander Nevsky church. It is the biggest and most famous in Bulgaria. If you go downtown, you will see this, as well as many other attractions. You should also see the history museum. It has many artifacts and works of art. (Especially this one Thracian gold mask. When it was discovered, there was an enormous line of people there to see it.) Also, the Mountain Vitosha is a very beautiful place that I would recommend you visit. Go to the area around the government building and the national theater in the downtown area. Watch out for thieves.

i don't know when you are going, if in the winter, go to sofia, if at summer, have fun at the beach at Golden Sands or Sozopol :)