Concierge.com's insider take:
No other island is like it. This Minoan capital was decimated by its volcano 3,500 years ago, which has led many archaeologists to suggest it may have been home to the lost civilization of Atlantis. Santorini owes its present peculiar, twin-peaked, arched shape—as well its famous black-sand beaches and the high, striated black-red-gray cliffs—to that cataclysm. The caldera, a seven-mile crater enclosed by the two arms of the crescent, is Santorini's defining feature and its harbor. Sunset over the caldera is the nightly big show, which is accompanied by a festive atmosphere wherever there's a west-facing cliff-side terrace (and there are a lot of them). If you're thinking of walking, the capital, Thira, is dramatically perched on a cliff, up nearly 600 steps (alternatively, catch a cab, a cable car, or a mule), and is very gorgeous but very spoiled by tourism. The center of gentrified, controlled tourism is breathtaking Oia (a.k.a. Ia), clinging to the cliffs on the northernmost edge of the caldera, its 19th-century merchants' villas and restored troglodytic peasant houses spread around the ruins of a 13th-century Venetian castle. This is where the posh, beautiful hotels and villas are, and it's likely to stay posh and beautiful thanks to zoning laws. Many beaches on Santorini, as mentioned, have black sand (you can imagine what this does to bare feet), and they are concentrated in the east and south. Kamari is the main one but not worth the journey—it's been touristed to death. Head north-east to Baxedes beach, just outside Oia, to avoid the crowds.