These cost nothing and carry a full day each. Build around them and the paid tickets become optional extras instead of a budget drain.
Alexander Nevsky Cathedral
FreeTipEntry to the vast neo-Byzantine cathedral is free and it opens daily from about 7am to 6pm, but buy the small photo permit at the candle desk if you want interior shots. The crypt houses Bulgaria's best medieval icon collection as a branch of the National Gallery, with its own ticket and separate entrance left of the main steps; it closes on Mondays. Come back after dark, when the floodlit gold domes are the city's best free photo.
Ancient Serdika Archaeological Complex
FreeTipThe excavated streets, baths and houses of Roman Serdika sit directly beneath the Serdika metro interchange and the open-air sections under the glass domes are free to walk through whenever the metro runs. Enter from the underpass between the Largo government buildings and read the info panels eastward; the fenced indoor section with mosaics keeps office-style hours, so do the free open ruins first and add the ticketed part only if a mosaic floor matters to you.
TipSofia's oldest building, a red-brick Roman rotunda from the 4th century, hides in the courtyard between the Presidency and the Sheraton, ringed by more Serdika ruins; entry is free and it works as a functioning church, so keep quiet and skip photos during morning liturgy. Look up for three layers of frescoes from the 10th to 14th centuries, then time your exit for the top of the hour to catch the changing of the guard at the Presidency entrance a few steps away.
Vitosha Mountain and Aleko
FreeTipTake bus 66 from the Hladilnika terminus to the Aleko hut area at about 1,800 metres, or ride the Simeonovo gondola when it runs, typically Friday to Sunday outside the December-to-March ski season, daily during it. From Aleko the summit of Cherni Vrah at 2,290 metres is a two-hour marked walk with a kiosk on top serving hot bean soup; check the lift status online the night before, carry a warm layer even in August, and start morning hikes early because summer thunderstorms build after lunch.
National Palace of Culture (NDK)
FreeTipThe hulking 1981 concrete congress hall anchors the southern end of Vitosha Boulevard behind a long park of fountains, skaters and open-air cafes; the park is free and best at sunset when the mountain glows behind the building. Check the NDK box office or site for cheap same-week tickets to concerts and Sofia Film Fest screenings in March, and climb the pedestrian bridge over the boulevard for the best photo of the axis toward Vitosha.
TipBuilt in 1576 and attributed to the great Ottoman architect Mimar Sinan, this is Sofia's only working mosque, and visitors are welcome outside the five daily prayer times; entry is free, shoes come off at the door, and women should carry a headscarf. It stands within a hundred metres of the synagogue, the cathedral-facing churches and the former mosque now housing the Archaeological Museum, so string all of them into one short walk locals call the square of tolerance.
TipThe 1909 Moorish-revival synagogue is the largest Sephardic synagogue in the Balkans, crowned by a 1.7-tonne brass chandelier, Bulgaria's biggest. Visits run on weekdays and Sunday for a small fee of a few euros, and the entrance is a side door where you may need to ring the bell for the caretaker; it is closed to tourists on Saturdays for Shabbat. The small museum upstairs covers the rescue of Bulgaria's Jews from deportation in 1943.
Ivan Vazov National Theatre and City Garden
FreeTipThe 1907 national theatre presents Sofia's grandest facade, Viennese columns and gilded pediment facing a fountain pool across the City Garden, where pensioners play speed chess at the fixed tables every afternoon, free theatre in itself. Performances are in Bulgarian, but tickets start at only a few euros if you want the interior; otherwise come around golden hour, when the facade lights up and the garden benches are the best people-watching in the centre.