If you enjoyed Stoupa, you'll also enjoy Finikounda, mentioned by MariaH. The differences are that Finikounda is an all year round farming community, and (especially in peak season) is primarily a Greek holiday resort. Outside July and August it has lots of beach space and visitors are campers (mainly from Holland, Germany and Austria) about 50 to 100 Kosmar clients (like Stoupa, only fewer numbers), about 100 Finns and Swedes occupying hotels, and 50 sailors and windsurfers.
Beaches have less surf than in Stoupa, and are much longer. You'll get a good description of Finikounda looking at Dion-Zois web site -
http://dion.zois.googlepages.com/english . They run five small holiday apartments about 3 min walk from a long beach, and 7 min walk from town centre. They also have links to Gordon and Wendy's web site . . .
http://www.greekrooms4u.com/ . Wendy was the Kosmar rep for a few years . . . they now book accommodation and arrange transfers for direct travellers.
Of the islands mentioned above, Skiathos is very pretty, but the beaches are much busier and more crowded than either Stoupa or Finikounda, with rows of brollies and sunbeds occupying a lot of the space. Parga is easily the biggest resort mentioned, strongly oriented to the British visitor with lots of tourist shops. Valtos beach is big, but it's a bit of a trek over the hill from the town, and pretty busy.
My web site includes a tourist guide to Messinia (SW Peloponese). It's oriented a bit towards sailors. You'll find further gems described there - such as Yialova, on the Bay of Naviarino, just of Pylos. But that may be too quiet for you - miles of sheltered beach, sand dunes, tamarisk shade (no need for beach brollies), six tavernas, three cafes, good walks . . .
Google 'jimb greece' to find the site.
JimB