As a couple, Akrotiri Lighthouse works.
The anti-Oia sunset. Instead of jostling a thousand phones at the castle, a handful of people sit on a bare, wind-scoured headland at the island's southwestern tip and watch the sun drop straight into open Aegean — no caldera, no village, no commerce. The trade is specific: the 1892 lighthouse itself is closed to the public, there is nothing to do but watch and feel the wind (bring a layer, it's genuinely gusty out here), and it's a real drive from anywhere with your own wheels required. What you get for that is the one sunset on Santorini you can have almost to yourself, framed by sea rather than rooftops.