We did a 14 night Eastern Med cruise back in 2019 that included both Mykonos and Dubrovnik. While we loved Mykonos, I would pick Dubrovnik without a shadow of a doubt - especially as you already going to Santorini. The natural beauty, the architecture and the tragic recent history of Dubrovnik drew us in. I'm not a GoT fan, but my husband got a kick out of seeing some of the sites where the show was filmed. I would love to go back to Croatia and spend some more time exploring the rest of the country. I hear Split is lovely.
Mykonos is a gorgeous island and we enjoyed our day there, just walking around the old town and having lunch. But it wouldn't draw me back for a longer stay as Dubrovnik did.
I've never been to Crete, so I can't help there. I did notice that the 6/18 cruise is also going to Kefalonia. This island was a pleasant surprise for us. It was just lovely, but in a completely different way than Santorini and Mykonos. The call it the "green" island as it's heavily forested with some amazing waterfront views. Someone on our tour said it reminded them of Lake Tahoe. We had a lovely day there, exploring the island, visiting a winery and an old church, and taking a boat ride through a gorgeous cave among other things.
Whatever you decide, enjoy! You really can't go wrong, and you will probably end up wanting to go back, so you can explore whatever you miss on this trip

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