I'll be on the Oosterdam in October, when in Naples, think Herculaneum instead of Pompeii, you can do it yourself by train. Depending on where you're staying in Rome, you can also take the train from Vatican station instead of the main station which is further out, the train goes every 30 minutes to Civitavecchia and is a 45 minute ride.
Hal excursions depend on what they offer, a walking tour you can do yourself.