Sydney, Australia, is the the only port I've been in that once you embark, you cannot leave the ship.
Ravenna is not that close to the port area. The center of town is about 8-10 miles from the port.
I'm sure once you are allowed to embark, around 11am, you would be able to disembark and head into town, but you have to check sailaway times, and all-aboard times before you leave the ship. We were there 2-3 three years ago, and there were not plenty of taxis in town, so I would be concerned about getting back to the ship without pre-arranged taxi.