Sorry aawa, cannot help with other countries. I fell in love with Greece as a Teen, couldn't go until my 60s. Once there, I knew I had to return -- & June will be Trip #12! But since I'm not affluent (blame a long career in nonprofits!) I only manage to go by taking along 3-4 "newbie"Travel Pals... and by foregoing trips elsewhere (Hard for a Sagittarian!! travel is in our stars!!) Thus, aside from 2 London-Paris trips, no Euroknowledge in many decades.
But I do think you are too easily discouraged re Milos-Sifnos. If you can find lodgings, these 2 offer SO much! STUNNING Scenery. I hate the word "quaint" BTW (It conjures up picture of a tourist examining "locals" in "native costume") ... but both have deep cultural traditions and no "resort" affluent-vulgarity. Here are some reasons I love them:
SIFNOS -- until 1960s, NO roads between villages; people walked or rode donkeys on flagstoned paths, or sailed a boat. Now there are (small) roads, but paths are still there. In late May I plan to walk one from hilltown of Appollonia down to Chora overhanging the sea; a white vision almost unchanged in 400 years. In 19th C, Sifnos was THE pottery Center of Greece (made 90% of all kitchenware); it's now revived, & on Platy GIalos beach u can visit 2 potter studios. Sifnos was home of Greece's most famous chefs; it still has its special dishes. Below my hillside patio, every morning locals tend terraced gardens before going to work.
MILOS -- MORE stunning beaches than any other Isle!!. Volcano eruptions just outside, not in the middle like Santorini, so island rim has geological wonders! A day sail all-round or half-round the island rim (I recommend the latter, since it gives more time for going ashore) is a treat your boys would LOVE. Boats line up portside in Adamas town at dinnertime, u inspect & choose. I recommend the large Catamaran (14 people capacity) rather than tiny boats or big party boats. They go out about 10 AM, have lunch & swim on a cove beach... you can go on rubber rafts into natural volcanic arches/caves ... a great treat! Back by 6 pm.
As to yr alternatives: Italy has so many fewer Beach areas than Greece-- and they can be reached by car & busloads -- they're always desperately overcrowded in High season; Croatia is less besieged but from reports by friends, I think many beaches are on islands without so much to offer. And you know something, aawa ... don't make the mistake of having a Vision of the Perfect blockade you from the Really Very Good.
PS: Hydra is a no-cars Isle, not much to offer aside from a very attractive harbor,no beaches to speak of. Gets a lot of business mainly because of Rick Steves book & do-ability as day-trip.
PPS: If you want a wonderful beach stay that FEELS like an island, on a GReek beach far removed from the Madding Crowd, here is my secret Gem. Far southwest coast of Peloponnese (you would fly into Athens, domestic flight to Kalamata, rent a car there for a 45 minute drive). It is ZOE RESORT ( thank God, NOT really a resort, just a wonderful family-run place, Some marketer 2 years ago told them to change the name, blessedly nothing else changed). Website: http://www.zoeresort.com Directly on a stunning beach, in Gialova, a teeeeny village (3 blocks long) just north of Pylos (gorgeous town, laid out by FRench). Amazing loooong sandy beach goes right to a wildlife refuge, and famous bay of Navarone (Castle ruins for boys to climb!) Zoe fronts on its' own shady grove of palms, beneath which you can sit in deep wicker chairs, and read all afternoon. Behind the hotel is a large pool, and another building with their apts. In High season, many youngtsters for yours to play with. After 16 days of showing a great time to 4 SUPER-energetic travel-pals, I escape here, to relax & recuperate. At night, I dine among the palms, lit by Japanese lanterns. Aaah.
If this does not please, I truly cannot help you.