There is a reason why Santorini is filled with tourists at all times of the year: it is absolutely beautiful. The colors against the white buildings, sitting on the cliffs overlooking the water. The sunsets are like nothing ever seen before. It may be overridden with tourist, but it is a must for anyone traveling to Greece.
Oia is the picturesque town that is in so many photos. Definitely make sure to visit the town during sunset. But the rest of the island shouldn't be ignored either. Fira is the larger town and is where you can capture a photo of Oia with the sunset in the background.
From Oia, you can walk down to the Bay of Ammoudi where you can get fresh seafood pulled right from the ocean.