Most people assume that from Athens, going for a swim in the Mediterranean is a simple task. This isn't completely true. The closest beach from downtown Athens is about a 30-minute drive, and this beach is not that great. If you're taking public transit you can be looking at more than an hour before you see the beginnings of beach life, up to two hours before you start seeing really nice beaches. If you have the opportunity, make sure you get in all your beach time on an island, because Athens really isn’t a beach town.
If you are desperate for a swim, then heading to Glyfada is your best bet. Found along the coastal road towards Sounio there are many beach options. It’s fairly easy to get to using public transportation, but it is fairly slow. You will find a few beaches before you hit Glyfada, but keep in mind the further you travel from the somewhat polluted port of Piraeus the better. Keep in mind that some beaches are pay beaches; you can be charged up to 50 euros per person, so a quick dip might cost you.
If you're feeling a little more adventurous, you can find some very nice beaches in the Nea Makri area. These beaches are very popular with the locals for a quick getaway. To get to them you need a car, and getting there can be a little challenging, but they’re well worth it.