Santorini, ancient Thera, is unique among the Greek Islands. Most of its buildings sit on the brink of an ancient caldera rim, overlooking a circle of sea. Its beaches are made of fine black sand and you can visit a still smoking crater in the middle of the island. As if this were not enough, on the south tip of the island you can find a Minoan-era city, still under excavation. For thousands of years it lay buried under the ash of the cataclysmic eruption that transformed the island from a circle to a ring (and possibly hastened the end of the Minoan civilization.)
A most interesting place--not to be missed! Beware the summer tourists however.