We walked into the city and took the hop on hop off bus. In the time we had available, that is all we could absorb. If we ever make it back to Naples, we will probably take an independent tour of the countryside.
We bought a city bus ticket and took the bus from the port to the walled city. The bus terminal is right outside the cruise facility. The old walled city was fascinating. My wife expected to see a museum and she was just amazed to see a living city instead. I suggest starting early and wearing some very comfortable walking shoes. There is a lot to do and experience. We did not have time to walk the city wall, but I think that is a must do if we ever manage to go to Dubrovnik again.
We stayed in Venice overnight. We bought a 2 day ticket to ride what I would describe as a water bus. It is the best way to get around in the city. Again, there is too much history to absorb in the time available. We concentrated on St. Mark's Square, taking some time to walk the narrow streets in the vicinity. We bought a city map with the "bus" tickets and that proved essential. If you do some research on the city before your tour, you will have a much better idea of what to see. The locals commuting on the boat were happy to tell us about some of the points of interest as we went down the Grand Canal. We ate a lot of gelato to be able to use the bathrooms at the restaurants. The pay toilets were up to 1 1/2 Euros each.
We spent 4 days in Barcelona after the cruise. The Gray Line hop on hop off bus is a bargain for seniors, with a 30% discount. I had a 2 day pass for the same price as others paid for 1 day. Wherever we went, part of every day was spent on las Ramblas. This is the main tourist area from the center of the city to the waterfront. Not to be missed. We went to the Cathedral several times and enjoyed walking through the small streets in the vicinity. Stay where the tourists are and be extra careful after dark. We came close to being mugged on our third night when we became a little overconfident.