I had been to
Santorini as a college student for the graduation excursion about 10 years ago and last summer I was there for my honeymoon (I guess, the nicest memories of my life are from Santorini!).
I love this island, I only wish I was not hundreds of miles away and could visit it more often (but New York- Santorini is a long way, don't you thing?).
Oia has the best sunset in the world! It was so romantic to see couples hand by hand watching the sunset and artists drawing it. It is amazed how cruel, busy, modern people can get so touched by just one sunset!
Beaches are not the best in Greece, but when you are there, I least you care abous is swimming.
You will have so many other things to do. Try the Volcano Tour, visit Thirassia (a remote island opposite Oia) and hike along the vineyards. Santorini has pretty good wine.