Santorini is one of the great natural wonders of the world, and its main attraction is the landscape and seascape of the island itself. The configuration of the present, roughly semicircular island is the result of an enormous volcanic explosion which occurred probably around 1630 bce, literally blowing the top off the island and changing what had been a typical half-submerged mountain of an Aegean island into a flooded crescent caldera, in the middle of which a few small smoking islands still bear witness to volcanic activity. Some have speculated that this event was the inspiration for the myth of Atlantis. The towns of Fira, Ia also known as Oia and Thirasis cling to the steep cliffs facing into the caldera bay. Tours to the central "smoking" islands are readily available and one can see and feel steam vents and recent 1950s lava flows.
Another popular reason for coming to Santorini are the legendary sunsets, some of the most spectacular in the world. Ia is one of the few places on the island which is both close to a sea and offers a good view to a sunset over the sea: in other towns, the sun disappears behind the volcano.
Additionally the town of Fira is stunning.
Be sure to explore the areas outside of the towns. There is beautiful countryside where tradition still survives. Cave houses both abandoned and occupied, gardens, vineyards, small family business, and tiny churches are there to be discovered.
Santorini ranks among top destinations for wedding celebrations for at least 4 years -- primarily for sunset and peace, like those in Oia. Couples often arrive with few friends, stay in Ia places like Fanari Villas. Groups often arrive in the beginning of the week -- judging by demand for cabrios and number of corteges seen on Mondays compared to weekends.
While the island is full of medium- and top-cost hotels and villas, there are still lots of abandoned caves and modest private houses where noone seems to live for a long time -- even in western Oia where every inch seems to be occupied by some villa. And this doesn't seem to change for years, judging by 2001-2005.