Airport: there are daily flights to/from all major European cities, either operated by the corresponding national airline or by Bulgaria Air, Wizz air, Ryanair. The metro station (at Terminal 2) connects the airport to the city centre within 20-30 min (see below for public transport description). There are free shuttles between Terminal 1 and Terminal 2 (5 min). Late night arrivals (after 12 am) can take a taxi from each of the terminals, cost to the centre is ~20-30 BGN (10-15 EUR).
Bus and train stations: there are daily buses and trains to/from all major Bulgarian towns and neighboring transport hubs (Belgrade and most of Serbia, Macedonia, Thessaloniki and Northern Greece, Bucharest and South Romania, Istanbul and Western Turkey). The main bus and train stations are next to each other and share a metro station and a number of buses and trams (see below for public transport description). The city centre is 5-10 min away by any transport, including taxi (cost to the centre ~10-15 BGN).
The metro system is integrated together with all the buses (electric buses are designated with blue on maps, ordinary buses with red) and trams in Sofia with a unified (physical or virtual) ticket. A single ticket (valid for one trip with change of transports within 30 minutes of re-entry) is 1.60 BGN (0.80 EUR), and the daily ticket is 6 BGN (3 EUR). The schedule of all public transport is available on google maps, which is the simplest way to navigate. You can also visit Sofia Public Transport.
Physical tickets can be bought in the kiosks inside a metro station or from the bus/tram drivers. The virtual option is simpler and recommended: just tap in your bank card (or mobile phone) at the entrance of the metro station/bus/tram. The system will automatically calculate and choose the cheapest option for you (e.g. the 4th single ticket is upgraded to a cheaper daily ticket, 5 daily tickets get discounted to a weekly ticket, etc).
Main building of Sofia University, "the rectorate" - 15 Tsar Osvoboditel blvd.
Conference hall "the Egg" (see below) is situated inside Sofia University's main building, on top of metro station "SU St. Kliment Ohridski" (lines 1 and 4) and 2 min walk from metro station "Orlov most" (line 3). There is also a bus stop for a number of bus lines stopping on the side of the building, which is clearly visible and easily identifiable from a distance.
While the building is hard to miss, finding the correct entrance requires more attention. Note that the main entrance from the facade is closed, so you need to follow the red arrows on the map below and enter the building from an internal courtyard.
The entrance of the underpass as seen from the exit of the metro station.
Entrance of "the Egg" lecture hall.
Rastaurant NOAH+359: 4 Raiko Zinzifov street
It is situated in the Medical University area ~15-20 walk from the centre. It is close to metro station "Medical University" (line 3) and bus stop "Umbal Aleksandrovska" (lines 1, 2, 8, 9 and 72, 604).
The most popular central area of the city (with a wide range of hotel and airbnb options for all budgets) is situated between the metro stations "Serdica", "NDK (National Palace of Culture)" and "SU Sv. Kliment Ohridski". Most sleeping options within this area will also be within a walking distance from the conference venue, which might offer additional convenience.
The central area (see above) also offers a range of local and international restaurants and bars, of varying quality and prices. Check online and ask some of the locals for recommendations!
The central sights in Sofia are all within walking range from one another, and a free walking tour offers some basic historical background at a leisurely pace.
Culture: A number of art galleries and museums are a few minutes away from the conference venue, such as Kvadrat 500 and the archaelogy museum. The Sofia opera and ballet calendar includes many international titles. Search online for live music and other cultural events during the conference period!
Hiking: Vitosha mountain is immediately reachable by public transport - see more information here or ask the locals for recommendations.