We did Rome to Ravenna on Constellation last summer. If you are from the US, there a typically direct flights from the east coast to Rome. Spending a few days at least in Rome is a must. The cruise port is about 90 min from Rome so need to factor that into your travel plan. The mosaics in Ravenna are absolutely stunning ( we spent a whole day touring there, having been to Venice previously), but if you have never been to Venice, it would be my recommendation to head there after disembarking in Ravenna- will need arrange some kind of transport from port to Venice ( celebrity, private or train) as it is a distance. Many hotels available in Venice and then would
fly back from Venice.