What's a good 3-day itinerary for Santorini?

Day 1 stays in Fira and the caldera rim. Start at the Museum of Prehistoric Thera by 9 AM, when the AC is still winning against the heat and you'll have the Akrotiri frescoes to yourself for 20 minutes. The Spring Fresco room smells like old plaster and cool stone. Walk south along the caldera path to the Cathedral of Candlemas of the Lord, built in 1827, where the interior is dim and the incense lingers. By noon, eat at Nikolas Taverna on Erythrou Stavrou, a no-view place where locals go for the 12-euro moussaka plate and cold Mythos. The food is better here because they aren't charging you for the sunset. Afternoon, take the 587-step path down to Fira's Old Port. The donkey smell hits about step 200. Take the cable car back up (about 6 euros, 3 minutes) unless your knees are iron. Sunset from the caldera path near Three Bells of Fira, the 1757 blue-domed church you've seen in every photo.

Day 2 heads north along the caldera. The Fira-to-Oia hike is 10.5 km, takes about 3 hours, and drops you straight into Oia from the south. Start at 7 AM before the heat turns the exposed trail into a griddle. The path runs through Imerovigli, the highest point on the caldera rim at about 300 meters, where the wind picks up and the volcanic rock crunches underfoot. Bring 2 liters of water minimum. You'll arrive in Oia by 10:30 AM, legs buzzing and ready for a cold frappe at Lolita's Gelato on the main marble path. After noon, visit the Museum of Ancient Greek, Byzantine and Post-Byzantine Instruments, founded in 1984. A single room of reconstructed lyres and Byzantine bells. Lunch at Lotza for grilled octopus at about 18 euros, with a caldera view that earns the price for once. Skip the Oia sunset crowd at the castle ruins. Watch from Ammoudi Bay instead, the tiny fishing port about 300 steps below. Same sun, 200 fewer people.

Day 3 covers the south. Rent an ATV from Fira for about 35 euros per day, or a compact car for 50. Drive 15 minutes to Akrotiri, the Minoan settlement buried by the eruption around 1600 BC. The covered site opens at 8 AM. Go early. By 10 AM the tour buses arrive and the walkways trap the heat. The preserved two-story buildings still have their clay storage jars in place, and the dry volcanic dust gets in your throat. Red Beach is a 10-minute walk from the site, down a loose-gravel path that is not flip-flop territory. The red cliff face radiates heat like an oven wall. From Red Beach, drive 20 minutes east to Perissa, where the black volcanic sand is hot enough by noon to burn bare feet. Swim, then eat a late lunch at Tranquilo on the beach strip for grilled sardines at about 10 euros. Afternoon option: Ancient Thera sits on the ridge 370 meters above Kamari, with Hellenistic inscriptions and a view stretching to Anafi island.

Santorini's buses run from Fira's central station to most villages for 1.80 euros, but service drops to once per hour after 8 PM in summer. Taxis are scarce, so plan your return from Oia carefully on Day 2. The island is 18 km north to south. Solo-traveler safety sits at 8.4/10 (see the TTDI Solo-Safety Index), and walking alone after dark is low-risk. Tap water is desalinated and safe but tastes flat, so buy a 1.5-liter bottle for 0.50 euros at any minimarket. Mid-September temperatures hover around 25°C with humidity near 70%, which means cotton breathes better than synthetics on the caldera hike.