In summer, Santorini (the colloquial name for Thira) teems with tourists. But September brings fewer crowds and better weather. Explore this volcanic island on foot with a hike from Fira to Oia. Arrive in time for the sunset, a spectacle of dimming light that casts prisms over the white-washed buildings and cobalt-domed rooftops. During the day, lounge at Kamari, a black pebble beach hemmed in by the sapphire-colored Aegean Sea.