Delos, reachable from Mykonos, and from neighboring Tinos & Naxos in high summer, is one of the smallest if not the smallest of the Cyclades.
In its hey day Delos was the political, spiritual and economic center of the Aegean with free port status and large population.
Revered as the birth place of Apollo and Artemis by their moon goddess mother Leto, (also pursued and impregnated by Zeus disguised as a swan and immortalized on canvas by Michelangelo) the island boasted an oracle second only to Delphi and a splendid and famous temple of Apollo funded by the combined mainland city states. That was then.
Today the island is uninhabited and has been under excavation by the French School of Archeology since 1873. The ruins cover an area exceeding 1 square km and several hours are needed to view them only if researching your dissertation. An hour or two will be more than enough for most people because when you have seen one pile of rubble you have seen them all. The island itself is 5.5 km long and 1.2 km wide and its west coast had three small harbors: little, sacred and commercial. Its still commercial! I am sure W. C. Fields would have something memorable to say about Delos and have no doubts that P. T. Barnum would look enviously at the present day operation. Still, its considered a major site but in my view is not worth the money. But go anyway if only for the caique to and fro expereince.
Unfortunately as you swelter in the hot sun of Delos, looking at a lot of rubble which they have tried to monetize, should you feel the need for shade, their isn't any except in the small hokey museum and the snack bar, both of which can be very overcrowded.
Warm soda, high prices, no water fountain and to top it all off toilet facilities are woefully inadequate and the staff harried. When you add up the cost of getting there and back plus the entrance fee, I, at least, felt disapprobation towards the authorities and 22 euros out of pocket. The whole place feels artificial, as though trying desperately to (unsuccessfully) justify itself and its admission fees. I fail to see what all the fuss was about. The view up the stairs from Mt. Kythnos was nice and getting there in the boat more memorable. In antiquity, on a good day, up to ten thousand slaves could be sold on Delos. They moved masses of people through Delos then and they move masses of people through Delos now! I am not annoyed by one or two people in my view screen but several hundred is another matter! My view, and I think the view of the authorities, is that most people who go to Mykonos will visit Delos as a matter of course, so don't rock the boat to Delos and visit off season only if you have nothing better to do. Read more about Delos.