We have been to a few of the ports you posted. Naples you must go to Pompei it will bow your mind. Dubrovnik is best walking on your own. Most of the tours of the center are this way and stop for lunch of local fish. You can buy a pass to walk around the Castle. Katakolon is very small and full of local trinket & trash stores. The tour go out to the Olympic site worth the price and info. Santorini can be done by just walking around yourself. We had the best view at the top in a local coffee shop overlooking our ship it was the best. Messina we took the tour to Mt Edna it was ery ineresting. Wish we had more time to spend there. We went On Costa cruises and really enjoyed ourselves.