The old town and Kapana quarter are within easy walking distance of each other and of the central pedestrian zone. Most of what visitors want to see is concentrated in this compact area, making Plovdiv a very walkable city for tourists.
Taxis are cheap by Western European standards and useful for reaching Plovdiv Airport or outlying sights. Use the meter or agree a price before getting in; avoid unmarked cabs at the train and bus stations.
Public buses cover the wider city and are very cheap; the system is not intuitive for non-Bulgarian speakers but Google Maps can navigate it.
By foot: the old town cobblestones are steep and uneven — good walking shoes are essential. The main pedestrian street (Knyaz Alexander I) and the central plaza are flat and easy to navigate.