Serres rewards a half-day on foot. Start at the Byzantine acropolis above the city — the tower of Orestis and the walls give a clear sense of the medieval fortress town — then walk down through the old neighbourhoods to the Ottoman mosques and the Archaeological Museum, which occupies a converted 16th-century covered market. Plateia Eleftherias, the main square, is the natural break point.
The hills around the city are also part of the experience: Lailia, half an hour to the south, is the closest forest and a popular weekend escape, and the early-Christian basilica at Amfipoli is an easy drive away.