In September, my wife and I spent a week long vacation on the island of Skiathos, with a day trip to Skopelos and Alonissos. Islands are these days mostly known as Mamma Mia movie islands, but also have plenty of nice beaches and walking tours. The southern part of the island is covered by good bus lines and water taxies, but you need your own transportation (or take a boat tour) to visit northern beaches or Kastro. At the end of south bus line is Koukounaries, salt lake biotope. The weather in September was far from that on southern Greek islands - good enough to go around, however, for baking on the beaches, you'd better go there during the summer.
Skiathos also has the shortest commercial landing strip on Earth, so make sure you get there by an airplane. :-)
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