In 2007 I went on a Contiki Greek Island hopping tour with a friend. It was a thirteen day tour with three days each on the islands of Mykonos, Santorini and Ios with the balance being spent in Athens and one day lost to travel. While this tour is pricier than a vacation to the Caribbean, I still highly recommend it to those people who like warm destinations with great beaches.
In my series of Cuban articles starting with A Guide to Vacationing in Cuba, I talk about how wonderful a Cuban vacation is with regards to the beach and the all-inclusive resort. While the beaches in Cuba are wonderful, I have yet to encounter any that are nearly as nice as the ones you experience at the Greek Islands. Particularly those of Mykonos and Ios. The sand is generally white and the ocean in the bluest of blue I have ever seen. There is never a problem with rocks on the beaches and the weather was perfect everyday that I was there. I went in late August so that may be a function of the time of year that I went, but if you're interested in consistently hot and sunny days then the Greek Islands make a great choice for a summer getaway.
I found Santorini's black sand beaches to not really be my taste and the island is much hotter and more desert-like than the others. Santorini is also considered a spot of romance for couples which also may have taken away from my experience as I was a single male at the time. Overall I still enjoyed the island, but less than the other two.
While the island-hopping tour is more expensive than an all-inclusive Caribbean destination, a Contiki tour of the islands is likely the best deal you will get. Amenities and restaurant choices on the islands can be somewhat limited, and if you go on a Contiki tour, your rooms are taken care of and most meals are too. When you get to Athens, some of your meals are provided.
The islands cater to tourists and generally are not great places to learn about Greek culture. Ios is known as being the party island but I found that only to be the case in terms of quantities of clubs rather than quality. Mykonos was actually the best party place of the three islands.
The disadvantage with a Contiki tour over venturing to the islands by yourself is that you will only see these three islands and possibly a fourth during one of the day excursions. If there is an island that you want to visit that is not on the itinerary, you'll probably have to go back after the tour is done since it is impossible to venture off from the group as going from one island to the next involves several hours of travel on a ship.
If you are interested in other Contiki tours, you can read my article Going on a European Contiki Tour by Yourself.