Just before Legraina, in the southern part of Attica (62 km from the centre of Athens) in the direction of Sounio, you will come across a sign that says "Property of KAPE". Turn there and follow the dirt road that will lead you to the most famous "hidden" beach in Attica.
A sandy beach and turquoise waters are its basic features that make you think you're on an Aegean island. If you visit on the weekend, be sure to arrive early to grab a spot, and if you prefer fewer crowds, come during the week.
The beach is not serviced, so make sure you have purchased the bare essentials for a day at the beach.