I found an article yesterday regarding 10 best Greek Islands. The link is very helpful. I wish I read this before I went to Greece last May. Regardless, take a look if you are planning to go there soon.
I would recommend people to go to Crete, because it is one of the largest islands in Greece. There are so many things you can do there, but it will take you at least 12 hours to get there by cruise ship from Athens. It is also obvious that you should go to Santorini, one of the best islands to see the sunset. However, it will get crowed during summer since it is one of the most commercial islands for tourists. I would suggest you travel there early May, which is not the peak season.
10 Best Greek Islands