The city itself is an important religious center, and many things to do in Sofia are related to worship. It's home to the Bulgarian Orthodox Church and the Alexander Nevsky Cathedral, one of the largest cathedrals in the Orthodox world, both of which should be on the top of your list of what to do in Sofia. It is internationally recognized for its architecture, frescoes, and size, and the cathedral is definitely one of the main things to see in Sofia thanks having survived two world wars.
The Boyana Church, one of the historical treasures in the city declared a UNESCO World Heritage site in 1979, is also one of the important attractions in Sofia. It's a small medieval Orthodox church, famous for the frescoes that decorate the interior walls of the temple and a visit here is one of the popular Sofia activities.
Other places to visit in Sofia include the public bathhouse, built in 1906, where some ancient Turkish baths used to be. Inside there are indoor baths and several rooms for body treatments which have since been converted into a museum. The building is located in the city center, opposite the Banya Bashi mosque.
The city is also very rich culturally, with lots of cultural stuff to do in Sofia, like visit the universities, museums, and other institutions. Highlights include the National Art Gallery of Bulgaria, the National Archaeological Museum, the National Museum of History, the National Opera and Ballet and the National Palace, among others. For leisure, it's best to stroll along Vitosha boulevard, the main shopping street and therefore, one of the busiest Sofia attractions most frequented by tourists and locals. Here you can find cafes, expensive shops, outlets and boutiques selling international brands.