We went to lake vouliagmeni our last day in Athens, the entrance fee is 8 euros which I think is reasonable-however it is 5 euros to rent a towel, which is ridiculous. Many locals there bring towels from home. The water was cold, but you adjust within about 2 minutes, and even in january there were a few people swimming (including me) :). Their are small fish swimming along side you and nibbling at your feet, which I didn't expect! There is a small dining area, a changing area, and hot showers. Definitely worth visiting!
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