Cannes is Cannes. If it's not the film festival all the glitter is gone. The port is pretty. Like a less attractive Santa Monica with Rodeo Drive shops. The best part of the port are the outlining areas. We visited Nice and Saint-Paul. Both are beautiful cities, but Saint-Paul was our favorite with it's old, beautiful architecture, winding cobblestone streets, shops, art and food.
Barcelona is magnificent! We took a private 4-hour tour and hit all the highlights but didn't leave any time to wait in lines to some of the museums. However, just seeing all the beautiful architecture was well worth the trip and maybe a return visit in the future.
Civitavecchia in Rome is the embarkation and debarkation port. There isn't much around the port and it is about an hour from Rome or the airport. Rome is wonderful if you have time and it's not crowded with workers and tourists (August is good because Italians are on vacation). So much to see and do with all the ancient buildings and art.