One of Europe’s oldest towns (settled since 5000 BC), Sremska Mitrovica was once Sirmium – one of the Roman Empire's four capitals and, at its peak, one of the largest cities in the world. You can visit what remains of old Rome here: excavated mosaic floors, rooms, baths and ancient heating systems are on display, along with a fascinating scale model of Sirmium.
Buses from Novi Sad run to Sremska Mitrovica (580RSD, 1¼ hours, hourly).
The Museum of Srem, a short walk from the Imperial Palace ruins, displays numerous archaeological findings from Sirmium, including a unique Roman sundial with the sculptures of Atlas, Heracles and Ifikles. A little further down the street, on Žitni trg (Grain Square), are the recently reconstructed open-air ruins of what was once ancient Sirmium's crafts quarter.