Of the northern Dhodekanisa, Kos, the second largest island in the group, after Rhodes and about 1/3 its size. It is the most fertile island, and also one with many sandy beaches, as well as some lovely forest, over which hovers a good mountain range for hiking, and a superb main city.
Rhodes town has everything from a Knights' Castle, a mosque, Hellenistic and Roman excavations through which one can walk scattered about the town, an old Ottoman quarter, to Art Deco buildings built by the Italians.
After Rhodes, Kos is the second most visited by tourists in the group.
Nearby Kalymnos and Leros are very different from Kos, and from each other as well. Kalymnos is a dry island made of limestone, though with a couple of fine green valleys, some good beaches, caves, good hiking, and a fascinating, colourful main harbor town with multi colored buildings, including some fine mansions, a town, which used to be a big sponge diving port.
Across from Kalymnos, the islet of Telendos is free of cars, but not of package tourism, which seems to find even the most out of the way places sooner or later.
Leros, with many deep inlets, sand and , pebble beaches with very clear water, is quite green and largely unspoiled by tourism, the lack of an airport a contributing factor. It also sports some fine Art Deco buildings.
Patmos, despite its being the second most visited of the northern Dodeka-nisa, has also kept its quiet (at least away from the port) partly due to the lack of an airport, but also because of its famous monastery of St. John, which has a subduing influence.
Lipsi, to the east of Patmos, is another green, unspoiled island with good walking, though hardly undiscovered; Arki, to its north, is tiny, with a deep yacht anchorage and a small fishing community, and even tinier, is Marathia, also with a yacht anchorage, and also a good beach. There's yet another yacht anchorage on the unspoiled, dry island Agathonisi, to the northeast, as well as some fertile areas amid the dryness, three villages, and an ongoing island music tradition.
The southern Dhodekanisa is dominated by the huge island of Rhodes, with its fascinating medieval city, its acropolis of Lindos, beautiful sandy beaches and partly forested hinterland. Nissyros, to the south of Kos, boasts a dormant, sputtering volcano, much green country along with rocks, castles, monasteries, villages, good walking, beaches, villages, freelance camping, is not in the lap of tourism, and hosts events from the Symi festival in August.
Tilos, to its south, is also unenslaved by tourism, and quite green , with good beaches, freelance camping, coves, a monastery, cave, and Knight's Castle. Just north of Rhodhes is Symi, with its harbor that has often been compared with that of Ydrha (Hydra), with its amphitheatrical harbor lined with neoclassical mansions, a waterfront also given national protection, and lined with cafes and tavernas.
The upper town is like a village, with folk museums; inland there are some wooded areas, monasteries, hiking paths, swimming coves, a fijord, and offshore islets. The all summer Symi festival is held here as well as on some of the other islands of the Dodhekanisa. The harbor of Halki island is compared with that of Symi (and Hydra), and, despite the lack of good beaches, the island gets a lot of tourists.
Another beautiful harbor is found on Kastellorizo, the easternmost of the Dodhekanisa., though the mansions up front are a fascade for many ruined ones behind, destroyed by an ammunition explosion in WW II. Still, there's a Knights' castle here, a Lycian house tomb carved into the cliffs, accommodation in unruined mansions, a Dorian citadel, no package tour presence and no good beaches.
The islands of Karpathos and Kassos lie far to the southwest, about halfway between Rhodes and Crete. Mountains, fertile land, good beaches, great hiking , and the famed traditional village of Olymbos are some of the attractions of Karpathos; Kassos is partly barren, partly fertile, with some good pebble beaches, a pretty fishing port and authentic Greek villages.