P&O do a tour called Rome on your own, which is basically a coach from the ship to Rome with no sightseeing etc. included (but you do get a map). Be aware though that Rome is a big city and the sights are fairly spread out. We did this tour the first time we went to Rome and to be honest we were quite overwhelmed by the size of the city and didn't really have the best of days. We've back since a few times and have done different P&O tours and had a much better time and got to see more of the city. Whatever you decide to do have a great time.
Hi you can easily get to the train station ( usually there's a courtesy bus to the town. The station is a few minutes walk from there and is very reasonable. If you want an actual tour, there will be taxi's at the port and probably mini buses which you could share. P+O may also just run a bus there, if you want to do your own thing. If taking a taxi or minibus, agree price beforehand.
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If you type 'Rome' in the search box at the top you will find lots of comments on how to do Rome by yourself.
Its quite a long way to Rome from the port so beware and do your sums first. Kind regard.