A high rocky headland (Mesa Vouna) separates two popular beaches (Kamari and Perissa), and at its top stand the ruins of ancient Thira. It's an incredible site, where cliffs drop precipitously to the sea on three sides and there are views of Santorini and neighboring islands. This group of ruins is not easy to take in because of the many different periods on view -- Roman baths jostle for space beside the remains of Byzantine walls and Hellenistic shops. One main street runs the length of the site, passing first through two agoras. The arc of the theater embraces the town of Kamari, Fira beyond, and the open Aegean. Greeks lived here as early as the 9th century B.C., though most buildings are from much later and date from the Hellenistic era (4th c. B.C.). You may not decipher everything, but do take in the view from the large Terrace of the Festivals. This is where naked lads danced to honor Apollo (inadvertently titillating, some of the graffiti suggests, a number of the spectators).
You can reach the site by bus, car, or taxi or, if you wish, on foot, passing on the way a cave that holds the island's only spring. Excursion buses for the site of Thira leave from Fira and from the beach at Kamari. Allow yourself at least 5 hours to view the site, if you walk up there and walk back down; if you come and go by transport, allow at least an hour at the site.