Asenovgrad (Асеновград) — a small city with most churches per capita in the country, close to Plovdiv at one of the entrances of Rhodope Mountains, famous for its Pamid red wine. Be sure to visit the Castle of Asen and Bachkovo Monastery to the south of the city.
Haskovo (Хасково) — go see the KDK club, worth having some beers with a friend
Hissar (Хисар) — a nice little town a bit north of Plovdiv, known ever since Roman times because of its mineral springs. The central parts of the town are surrounded by an impressive city wall from Roman times, which is still in good shape. You can easily visit the Balkan mountain range and Koprivshtitsa from this base.
Kazanlak (Казанлък) — city supplying much of world's rose oil production, and hub for visiting nearby Thracian tomb of Kazanlak, a UNESCO World Heritage site and the village of Shipka with its beautiful Russian-style church
Koprivshtitsa (Копривщица) — although its name might be difficult to pronounce for many foreigners, Koprivshtitsa is a beautiful old Bulgarian village — certainly a must to visit if you are in this area
Kurdzhali (Кърджали) — a city located below a huge dam, with many historical and ancient sites
Plovdiv (Пловдив) — the nation's second largest city which boasts a lovely shopping promenade and many parks, an ancient city with a preserved amphitheater, and many "revival" style Bulgaria homes. Also make sure also to take a side trip to Bachkovo Monastery which is about an hour away.
Sliven (Сливен) — well known for its permanent winds and blue rocks which can be observed from the city, also a good place for quality golf at a reasonable price
Stara Zagora (Стара Загора) — with more than 8000 years of history, this is one of the oldest continuously populated places in Europe. Center of booming industry, remarkable cultural life and numerous tourist attractions, though it is the linden trees which give the city its character. Don't miss the Roman Villa Mosaics, the Neolithic Dwellings, the Regional Art Gallery, the countless coffee-shops and art-galleries, which give the city the feeling of a small Paris.