Athens is the capital city with the presence of amazing ancient structures, e.g. Parthenon, Acropolis, Olympic stadium. And you will get numerous blogs and sites giving you the detailed ideas about how to see Athens in a few days.
Santorini is of course, the dream of the European holiday. Couples swoon over getting cozy in Santorini, rest of the people count the money and save up to see the most beautiful sunset along side the caldera. Again, you will get many articles, covering both expensive and budget options (though it is not that budget, frankly speaking), to enjoy this island in the Aegean sea.
I am going to tell you about the other side of Greece, you won't hear so often. However, they are amazingly beautiful, totally photography worthy, and unique to Greece. So if you are visiting for a slightly longer time, and have time after Athens or Santorini, or have the guts to ditch Santorini (not Athens, because most of your flights will reach Atehns, and it is not as expensive as Santorini is), here is your pick.
1. Meteora : The land of jutted out mountains and monasteries