Many travellers would agree that visiting Greece in May and June is your best bet to take advantage of the beaches and beat the heat and the crowds. During the winter, hotels and restaurants at many small island resorts close, so if you come early in the season, you'll get their full attention and experience legendary Greek hospitality.
You can visit the islands of the Aegean and the Ionian sea—Donoussa, Schinoussa, Kimolos, Halki, Kastelorizo—by ferry or boat. They resume operations after the winter with maximum capacity. Enjoy water activities at Perivolos in Santorini, Psarou, Paradise and Paranga in Mykonos, the Athenian Riviera, Lefkada, Rhodes, and Halkidiki.
September and October mark the end of beach season and prices drop, but the weather is still great and there is still plenty of sun. It's a great time to visit touristic places such as Santorini without the crowds.