There are hundreds of miles of coastline around Athens and Attica and so not surprisingly there are some very good beaches to visit. Most visitors to Greece think only the Greek islands have good beaches, they forget about the mainland never mind Athens.
There is probably every kind of beach you can think of with in easy travelling distance from Athens. There are rocky beaches, sandy beaches, organized, public, private, rocky coves perfect for snorkelling, you name it Athens has it.
As there are so many beaches in the area I can only write about the most popular and the ones I know best.
Glyfada is in southern Athens along the Sounio road (Posidonas road). Glyfada is actually an area very popular with tourists, many Greeks own holiday homes there, and has many beaches both sandy and rocky, usually organized. These beaches get very crowded (especially weekends) as they are so close to Athens, 20 minutes by tram or bus from the city centre.
Voulgameni is just south of Glyfada and is both very popular and organized, but if you carry on down the road towards Sounion (Posidonas road) for a few miles you will find some very quiet rocky coves with clear blue water for snorkelling. One of these coves even has a bar on it, obviously this cove is less quiet than the others.
Varkiza is big resort town a few miles down the coast from Voulgameni and has a huge sandy beach. The sea isn't that clear, but that is normal with sandy beaches. Posidonas road continues all the way down to Cape Sounion and you will find lots of rocky coves or sandy beaches. But the further you go out of Athens the more you will need transport of your own.
The coastline to the east of Attica have lots of sandy beaches, many near Marathon. Although the town of Rafina is quite a busy port to travel to the Cyclades islands it also has some good beaches and very popular with Greeks. To get out to the beaches in East Attica you will need a car or motorcycle.
To the North-West of Athens, towards Corinth on the Saronic gulf, are some less known beaches but well worth the travel. Just past the town of Elefsina, on the old national highway are many small beaches and only about 20 minutes outside Athens. Most of these beaches are rocky and do get busy at weekends. Some of the most popular beaches are in the small village of Neraki maybe because of its vicinity to elefsina and Athens or great views of Salamina .
Further down the old national highway (or use the main highway, the Olympia Othos, which you have to pay a toll) towards corinth are the 2 towns of Kineta and Agis Theodoros, both are connected and have kilometres of pebble beaches with crystal clear water.
The towns are popular with Greeks and many have holiday homes there. Out of the 2 Agis Theodoros is the bigger town and by far busier, the beach here has beach bars, watersports and some shops. Kineta on the other hand is much more village like with only a handful of shops but still gets busy at the weekends. Some of the beaches in Kineta are excellent for snorkelling with clear water and an abundance of marine life.