Cagliari is the main port in Sardinia. It is reasonably interesting. Probably the best way to see it is with the open-top bus tour, but if you happen to be there on a Sunday, the trip to the Poetto beach will take up rather a lot of time. Some good restaurants with authentic Sardinian food.
Very interesting city, but needs rather longer than the under 5 hours we had there. Probably a tour to Pompey or Sorrento would be best choices for a short Port stay. We wanted to do a trip to Sorrento which was originally advertised for this cruise but then withdrawn when tour booking opened.
Rome is, of course, one of the most interesting and exciting cities in the world. But the two hour drive too and from Civitavecchia severely limits what can be done. For example a visit to the Vatican Museum (which includes the Sistine Chapel) would probably be difficult in the time available. But good for a taster of Rome to whet the appetite for a future longer visit.
Again one of the most interesting cities in the world and, again, a 4 hour round trip from Livorno fairly eats into a day long port call. As with Rome, best used as a taster for a future lengthier visit. Coaches are not allowed in the centre of Florence so before you even start the substance of the walking tour there is a significant walk to get from the drop off point (and back to the pick up point).
Alternatively, some people on our cruise opted to visit Pisa which is closer to Livorno and I think they were able to get much more out of that trip.
A delightful Island and a port call is probably the easiest way for most people to get to visit it. Napoleon's villa is interesting but the museum attached to it disappointing since most of the significant items have been removed to other locations.