The island is a drowned volcano, and the two things worth building a day around, Akrotiri and the caldera-rim walk, are both about that rather than about sunsets.
Akrotiri
The Bronze Age town buried by the eruption around 1600 BC and excavated with its streets, drainage and two-storey houses standing, all under a modern roof, so it works in bad weather or midday heat. Near the south coast and well signed from Fira.
Fira to Oia caldera walk
Around 10 km along the rim through Firostefani, Imerovigli and below Skaros rock, on a mix of paved path and road, taking most people three to four hours. Start early: there is no shade for most of it, and buses run back from Oia.
Santo Wines
The growers' cooperative winery at Pyrgos, with a long terrace over the caldera, tastings of Assyrtiko and Vinsanto, olive oil tastings, a shop and a restaurant. Open year-round and bookable online, which you should do if you want the sunset slot.
Museum of Prehistoric Thera
In Fira, and the necessary companion to Akrotiri: the frescoes, painted pottery and the gold ibex figurine lifted from the site are shown here rather than there. Small enough for an hour, and indoors when the heat is bad.
Nea Kameni volcano boat
Boats from Fira's old port and from Ammoudi run out to the black cinder islet in the middle of the caldera for a walk up to the crater rim, usually paired with the warm springs off Palea Kameni. Wear closed shoes; the surface is sharp broken lava.