You may want to read my review here on the site of the Oceania Nautica cruise we did last August.
Ours included Barcelona, Marseilles, Monte Carlo, Livorno, Civitavecchia, Taormina, Amalfi, Valleta, and Athens.
We spent two days in Barcelona, staying at the Hotel Royal on the Ramblas. In Marseilles. we rode city buses and shopped. In Monte Carlo we caught the train to Nice and the beach there, but next time I'd suggest Villefrance instead. Florence and Rome you know. I could have spent more time in Florence. Beautiful city. And the Tuscan countryside. Amalfi is a quaint town built on the side of the mountain. Lots of shops, an old church, an sidewalk cafes. Some people take an excursion to Positano from there. Valleta is hilly, but you can walk much of it. We saw the churches, about 1 every block, and viewed a film of the history of Malta in a museum overlooking the harbor.
Our cruise was in August, and while Spain, France and Monte Carlo were moderate temperatures, it got very hot for the days in Rome, Florence, Amalfi, Taormina, Valletta and Athens. The locals had all gone to the countryside and the cities were not crowded. Taorminia is reachable from Messina. A resort town built up on the mountainside, with an old Roman theatre, lots of shops and restaurants, and the view of Mt Etna.