The only one I got to check out was Crete. It was pretty cool, I thought. Scenic island, lots of history. I'd love to do Rhodes -- tons of historic sights there, as well.
Of course, Santorini is the island you see all those posters of with the white, cube-like buildings overlooking the sea. Not sure what there is to do once you walk around and have checked out the views, though...maybe dodge cruise ship passengers...
I would shoot for getting off the beaten tourist track. So many cruise ships stop on the same islands, giving their passengers just a few hours there, you'll probably hate it when you're caught in their stream.
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"Life is not measured in the number of breaths we take, but by the places that take our breath away..."