We found Rome very easy. On the maiden voyage so not that long ago. Train was very cheep EURO 19 .50 CENTS RETURN FOR TWO PEOPLE.
At all the venues you mentioned - apart from Rome - you can have an excellent day by staying in the town,rather than taking a trip outside - unless of course you want to visit a specific nearby location. At Vigo a shuttle bus will take you into town and we enjoyed walking around the old part and the coastal area, where we had a an excellent lunch. At Lisbon you can walk into town and either walk round the centre or get a bus or tram around. At Cannes, you will tender off shore and can easily walk around town from there, either along the promenade, into the shopping area or upto the old church and town - you can do all three easily in a few hours. In Ajaccio you dock in the centre and can easily walk around town, which we found very interesting especially with the Napoleon connections or catch a local tourist trip. Although many would disagree if you want to see Rome, I would advise the one organised trip you take is to Rome, as this city like London is vast and you are much better served being shown the main sites and why risk the vagaries of local transport which could result in you missing the ship !
We were on a cruise a month ago and made our own way to Rome which was extremely easy...I am sure there were hop on hop off buses outside the terminii but the underground is very easy to use and the train ticket included the metro fares...we got the metro to the Colosseum and then walked from there ...the metro also goes to the Vatican area and St peters but we didn't do that on this trip as it was a Sunday and we weren't sure it would be open for visitors....Regarding Gibraltar there are numerous taxis/minibuses waiting at the port gate which is only a short walk from the berths and apparently they will do cheap tours wherever you want to go....depending on how fit you are it is a level walk into the town of about twenty minutes. Hope this helps a bit.
We went on gibralter trip with other people it was minivan it worked out a lot cheaper they took you right to the top which was some view went into caves which was great all monkeys running about hold on to your bags and cameras do not touch them our taxi driver had to break to off them up they were fighting over baby monkey they sometimes sit on your shoulder or head one sat on my husbands head he tried to stroke it the monkey thought he was trying to take his bannana off him watch your back its a nice trip with lovely view.
Rome is very easy to reach by train. There are hop on hop off buses near termini.Otherwise just choose a couple of sights to see if you are doing it on your own.