For me it is very difficult to determine which the best beaches in Greece are. There are so many different islands and most all of them are surrounded by beaches with breathtaking turquoise water of varying shades of blue and green When I think of Greece my memory drifts back to the fantastic colors I saw when the sunlight kissed the sea. If you need more ideas on Greek beaches here is a good selection.
During the summer months, the seas surrounding Greece become so warm that it is like stepping into a bath. I recommend floating on your back for ten minutes and allowing your body to completely relax.
If you would like to see some very unusual beaches, you will find them on the island of Santorini. Because Santorini is a volcanic island, the sand of its beaches is black as opposed to white. The islands of Rhodes and Kos also have fantastic beaches.
However, if you are staying in Athens I recommend a ferry ride to the island of Aegina. It is not far from Athens but while you are there you will truly feel that you have left the city far behind. Alternatively you can drive north on the coast road until you come to a super beach called Varkiza. You can also reach this beach using a public bus. What ever you do, avoid Alimos beach. Not only is it dirty, but there are many shady characters that like to use it as a place to hang out. To go there is to run the risk of being covered in oil or robbed.