Astir is the most exclusive summer beach playground (giveaway: there's a helipad, for the most elite Athenian commuters). It has all the water sports, shops and restaurants you could want (plus, oddly, a TGI Fridays). It's 19.5km south of Athens, 7.5km south of Glyfada. If it all gets too flashy for you, there's a nice public beach across the road.