Civitavecchia is the port city nearest Rome. Since we spent several days in Rome prior to a cruise back in 2012, we decided to explore this city.
Upon exiting the terminal, we found clear directional signage and a sign listing prices for the shuttle.
A tourist stand offered a handy map of the city.
There were vendors selling tours as folks exited, so we said a polite, “Thank-you, no,” several times and took off on foot.
We happened on a beautiful church to visit,
with amazing sculptured doors 
and stained glass windows.
We found the Fort of Michelangelo,
a public beach area,
views of the port and ships of all sizes,
and major stores.
We found a small shop with tables and chairs offering wifi with purchase. So we ordered our soft drinks and caught up on email before heading back to the ship.
I expected Civitavecchia to be a dirty, port city, but I was pleasantly surprised; however, if someone has never been to Rome, I would recommend an all-day tour to see as much as humanly possible. Rome is 90 minutes away from the port, so it would be a long, exhausting day, but one worth the fatigue.
Next port: Livorno, Italy for the city of Florence.