We know the area very well and are familiar with all the options. But, which is best is a totally personal thing. If you are interested in ruins Pompeii and/or Herculanium are great options. The Amalfi Coast is a wonderful place (we have spent two extended vacations on the coast in Praiano) but we are not big fans of the cruise line bus tours since they are not permitted to enter into Positano with a bus (they can just look from above). Sorrento would not be very high on our list although it does have some charm and is worth a visit if it fits into your itinerary. You failed to mention Capri which is another option. As to excursions, we have been to Naples about a dozen times on cruises, spent more than two weeks in hotels on the coast, and have never taken an excursion. We prefer to do our own thing. But, for a first time visitor, if you really want to see the Amalfi coast you should consider booking a private tour that is done in either a car or van so that you can actually drive into Positano and walk around the city. For Pompeii, you could take an excursion, or simply go to the Naples train station and take a train to the ruins where you could get an audio tour or join an English speaking tour (much cheaper than a cruise line excursion). Capri is very easy to do on your own since the ferry pier is only a short (about 10 min) walk from the cruise ships.