Ancient Eretria was a major maritime power with an eminent school of philosophy, and was destroyed in 87 BC by the Roman commander Sylla. Its scant but fascinating ruins lie scattered around town; pick up a map at the Archaeological Museum. Southwest of the ancient hilltop acropolis (north of the main road) are the remains of a 5th-century-BC theatre and a 4th-century-BC temple; the most fascinating site is the House of Mosaics (370 BC), with pebble mosaics depicting mythological scenes.
Other remnants include baths, walls, a gymnasium, and the Temple of Apollo Daphnephoros with its origins in the 8th century BC. Allow a couple of hours to explore the ruins.