The centre of Sofia is small enough to visit on foot. Otherwise there are numerous old squeaking trams making their way around the city as well as buses and trolleybuses (electrical buses).
A section of metro has been opened so far and there are plans to extend it considerably.
Public transport runs from 5:00 until 23:00 (buses) or 00:30 (trams).
Trams do break down sometimes. We almost missed the train to Plovdiv once because the tram simply refused to move despite numerous desperate attempts from the driver.
You can buy tickets at newspaper stands and larger stops as well as from tram and bus drivers. One trip ticket costs 0.50 leva, one day ticket for unlimited number of journeys within the entire network 2 leva. So if you plan to make more than 4 trips per day, get a day ticket.
Immediately upon entering the vehicle you need to validate your single trip ticket at one of the archaic punchers mounted on the walls. You need to use another ticket if you change lines.
Tickets are randomly checked by inspectors. You will be fined 5 leva on the spot for travelling without a valid ticket.