Historic site — Loutra, ~14km from the city
The Roman city of Traianoupolis was founded in the 2nd century AD by Emperor Trajan specifically for its thermal springs, and flourished as a spa town for over a millennium. A 14th-century Ottoman guesthouse, the Hana, still stands nearby on the old Egnatia road that once connected Istanbul to Rome.
The historic baths — two domed buildings dating to the 16th century, each with a thermal pool and changing area — sit within the archaeological site, alongside modern facilities built in the 1960s. The water itself emerges at 51–52°C, mineral-rich and mildly radioactive, historically used for both bathing and drinking therapy.
As of this writing, the hydrotherapy centre is closed pending a licence renewal, though the spring still flows and the archaeological site remains accessible — worth checking current status before planning a visit specifically for the baths.