Hi Taline. The ship provides pool towels, so no need to bring them with you.
I've heard Naples described as a starting point to do more interesting things...people tend to pass through rather than spend time there. And, there is so much to see, as you know from your research. You can take the train to Pompeii, the hydrofoil to Capri, for example. There are several posts in our Europe Port Chat section
regarding Naples, as well as a great thread on train travel in Italy
We did an NCL tour in Naples -- Flavors of Sorrento and Pompeii. It was pretty extensive -- in addition to time in both places, we had lunch at a family-owned farm near Sorrento. It was a great day and we saw and did a lot. Loved Sorrento! Pompeii is amazing, and somewhat overwhelming. Should you go, definitely read up on it before your visit.