All the organised beaches offer watersports activities during the sujmmer, for example you could rent a sea bike or a canoe or a jet ski. Some beaches like Loutsa and Varkiza offer windsurfing rentals. Another interesting all year option is to take a tour. There are a lot of interesting tours, you could take a bike tour, an Acropolis tour, a food tour or a cooking lessons tour. There are also daily tours to Mycenae, Epidaurus, Sounion and Delphi. If you would like to arrange an Athens tour take a look here.