Double up the Skiathos comments - walk through woods for 30 mins to the beach at Xerses. (left end of island) . . . Thats a secret by the way and I don't expect to see anyone else there now . . . . Beyond words - stunning, peacuful, hardly a soul - they are all on the other side of the island and getting all hissy with the sunbed salesmen.
Go to Crete - south side at Mirtos - drive along the coast to the west and there totally deserted beaches - plus one or two with naked Germans - not a pretty sight - give those a miss and find your own beach to enjoy. Or on north, get to Sitia, or way out to the east at Vai - palm trees and silver sand. . . .
Greek Cyprus as well of course - go along the Akamas peninsular on the north from Polis and enjoy.