Modern Eretria is a small, for Greeks mostly, resort with a population of 5000. Euboea its self is a lovely island with much history and beautiful and varied topography. You may read much more about Euboea (Evvia) here.
I decided to put a link to the island of Evvia reachable from the Greek Island menu as well as the Greek mainland menu so more people learn the truth about Euboea and what a great place it is to visit especially with a car. Islands can be a pain, they get crowded, boats get cancelled, ferry tickets cost money, food costs more and so on.
Visit Euboea and you will find that only a couple of hours from Athens (and there is even train service) exists a huge island with dramatic cliffs, long sandy beaches, and even natural phenomenon that perplexed the ancients and remain inexplicable to modern savants as well. Euboea is a truly fascinating place and off the beaten tourist track. Like Crete it is a place full of contrasts; beautiful mountains and lovely beaches, forests and farmland.
Ancient Eretria, famous in antiquity is of interest today too. Its excavations revealed a theater with steps leading down into an underground chamber used for rapid exits and entrances; and ruins of a gymnasium and sanctuary.
There is also a House of Mosaics from the fourth century BC which can be viewed if one asks the guard from the museum.
The museum is open Tues-Sun 8am-2:30; 1.50 euros.
Picture right from the museum, during battle and smitten by Eros, Theseus is abducting a smiling Antiope Queen of the Amazons.
There is also an agora and a temple of Dafniforos Apollo closer to the town. 11km west of Eretria on the coast there is a very nice campground (Milos Camping: 2211 060 460; fax 2211 060 360).