Archeological Site of Rhamnous — Photo courtesy of Nelly ParaskevopoulouThe archaeological site of Rhamnous, about 40 km northeast from Athens, is one of the most interesting in the area of Attica and definitely worth visiting. The site is open from 8 am to 3 pm except weekends, so you can plan this day trip during the week, leaving sufficient time to get there before closing time and enough time to wander around the ruins of this fascinating place with its enchanting atmosphere.
Rhamnous was a fortified city, built in the 6th century BC, overlooking the Euboean strait, with two small harbors on either side of the hill. The sanctuary of Nemesis,
Sanctuary of Nemesis — Photo courtesy of Nelly Paraskevopoulou the avenging goddess, is located here and played an important role in ancient Greek religion. After wandering around the archaeological site, you can walk down to the coast, provided you're wearing walking shoes. 
View of the sea from Rhamnous — Photo courtesy of Nelly ParaskevopoulouThis archaeological site is not much visited, so you can enjoy the peace and get the feeling of ancient Greek culture and worship.
On your way back, you can enjoy a lunch of fresh fish at the traditional fish tavern overlooking the small harbor of Agia Marina, very close to Rhamnous. Boats leave from here and go to the large island of Euboea. On the road back to Athens, you can visit the Archaeological Museum of Marathon, with objects from the Neolithic to the Roman age, including the Egyptian sanctuary, which have been discovered during excavations in the area. Also visit the Marathon Burial Mound, dedicated to the Athenians who fell in battle against the Persians.
If the season permits and you feel like a swim, the coast of Schinias, with the pine forest, is ideal. The coast of Marathon is popular for the many ouzo and meze restaurants, and has a lively nightlife during the summer months. The monastery of Saint Efraim, who performs miracles, is also an interesting place to visit. Stop for coffee at Cavo, close to Nea Makri, and enjoy the view and the desserts.
The port of Rafina is very close to the main road leading from Marathon to Athens, and is a small seaside resort with many fish restaurants. Ships leave from here for the nearby islands of Andros and Tinos. This area is very popular for vacations close to Athens, and is a favorite with locals and visitors. The area of Mati, just after Nea Makri, offers several good choices for accommodation: Mati hotel, Cabo Verde and Aquamarina on the coast, Myrto on the main road, and Avra and Akti in the harbor of Rafina.