Athens coastline is extended from Faliro until Sounio- and not mention the East coast (Lavrio, Porto Rafti, Rafina, Shinias, Marathon) of Attica, from where you take the boat to visit some Aegean islands.
Here are some recommendation for beach bars.
Bolivar Beach Bar (Kalamaki tram station). Located in Alimos, at “Akti tou Iliou”. When you get there, you can’t miss it. There is entrance fee here.
Akanthus (Zefyros tram station). Also in Alimos, it is also really popular among Athenians, also with an entrance fee.
Balux cafe, near Glyfada area.
Kavouri beach, accessible by bus, or by car only. Worth visiting, not very far from Athens, cafe and food in normal prices, no entrance fee.
Vouliagmeni public beach, no fees, everything free.
If you want to go even southern, you can visit Anavyssos, which is a fantastic beach.