Florence and Rome are not near the port, and I would not recommend trying to do them independently. Dubrovnik & Split can be done easily. If you dock in the commercial area of Dubrovnik, there will be a coach, probably at extra cost, to take you to the old town. If you tender, you will be deposited right in the old town. Barcelona has a lot to see so you should probably opt for a city excursion, but it is nice just walking up la Rambla from the cruise terminal. First time there, I would take the tour. I don't like Naples myself, but that's just be me. I've walked around there but wouldn't recommend it. If you want to see it, take the tour.
Hope this helps.