Skiathos is the most well-known of these islands and has at least 30 sandy beaches around the island. It is a gorgeous island, lush with green and vegetation, much like Mt. Pelion just across the sea and has the advantage of natural shade on the beach–and almost always a calm sea, being protected from northern winds by Mt. Pelion on the mainland, which makes it ideal for those who love swimming or sailing. In fact, out of the 37 islands and many mainland destinations I have visited, I can say with confidence that the best beaches for swimming are without a doubt on Skiathos, as well as the calmest water for sea chartering. Here is more on the island.
I was fortunate to live there from June 1995-September 1996 and have my picture perfect wedding in the courtyard of the 17th century monastery, Panagia Kounistria (the patron saint of Skiathos). It was on Skiathos that I greatly expanded my knowledge of Greek cooking with kyria Katina from whom we rented a house next to her amazing restaurant “Anatoli” (East). In general, Skiathos has phenomenal food if you know the right restaurants. Here is an article about traditional dishes and recommended restaurants
Although Skiathos, and mainly Skopelos, gained fame from the filming of “Momma Mia”, the islands are not nearly as well known in the US as the drier and much windier Cyclades (the group of islands containing Mykonos, Santorini, Naxos, Paros, Milos, etc.). Skiathos is, however, a popular destination for Europeans, so it can get quite busy in the summer from mid-July to early September, but as there are so many beaches and walking paths, one can always find a quiet spot even at the height of the tourist season. Here is another article with great photos and cultural information.