Welcome to Apollo Coast
The coast road (Poseidonos Ave, often just called Paraliaki) leads from Glyfada to Voula, Kavouri and then bustling and popular Vouliagmeni and ritzy Astir Beach. The coast is a bit expensive, but the further south you go, the cheaper and less built-up it becomes. Of note are the natural mineral waters at Limni Vouliagmenis and the gay and nudist beaches tucked in the rocky coves at Limanakia.
There's better (free) swimming northeast and east of Athens, at Shinias, Marathon and Vravrona; these take longer to get to and are best reached by car.