We got the train into Rome ourselves for a fraction of the cost of an official excursion. This was very easy as the station is across the road from the port and it is around 40 mins into the centre of Rome. Once in Rome you can get from St Peter's Square (The Vatican) to the Colosseum by bus which takes about 10 mins and costs 3 euros, then from there you can walk to the Trevi fountain which takes 10 minutes. Then walk to the main train station to get back to the port. The only thing that I would say about this is maybe don't go to the main train station in Rome as it is very big (it makes Euston look like a little village station) and as soon as you walk into it students practically pounce on you offering to take you to the platform that you need for a fee. Although this can be a good thing as the platform that we needed was about a ten minute walk! This is one station further than St Peter's Square, so try and make your way back to that one from the Trevi fountain. Fairly easy to do on your own and much much cheaper!