The lake at Vouliagmeni is a large mildly thermal therapeutic spring. The waters are beneficial for a variety of ailments and it's a uniquely majestic place to go swimming. Its properties were first discovered or written about anyway by the Romans in ancient times.
The rock formations, as you can see, are pretty interesting, and even more so as you swim between jagged pinnacles of granite, marble and quartz. The slightly brackish water feels like velvet on your skin.
Its a good place for children and the elderly as there are shallow areas. There are nice facilities including a restaurant and therapy center with plenty of of shade, tables and umbrellas.
The restaurant is a bit overpriced but you can bring in your own food too. It costs 3.5 euros to get in . The E5, Voula, express bus leaves from Akadimias St. every 1/2 hour, costs 50 cents, and is an air-conditioned Mercedes.
The Mediterranean Sea is just across the road with Lambros restaurant, a famous fancy, i.e. expensive, seafood institution. You can eat overlooking the sea watching folks go for a dip or even swim yourself.
There is a lake phone number: 210-89-62-237.
You'll drive by on your way to Cape Sounion and the Temple of Poisidon, a good double objective for your rental car or with your private taxi driver.