Although I haven't been to Civita (di Bagnoregio? is that what you're talking about?), we did stop at Viterbo on our way to Orvieto. Viterbo pales beside Orvieto, but it has a number of historical places to see, and is a charming, attractive town. It was relatively easy to park, and a level walking experience. Allow the most time for Orvieto. It's a nice introduction to the area that includes Tuscany.
Of course these things change, but we couldn't get any food after arriving late at the FCO Hilton. So eat before your check-in. Regarding lunch, it can take a long time to eat lunch it Italy. I'd be cautious about a serious sit-down on a day when you're trying to cram in your last day of touring. I wish I could tell you how to predict that a restaurant might offer "American-speed" service from the outside.
Edit: No one has yet mentioned the possibility that you'll get a car that makes it impossible to HIDE the vast amount of cruise luggage you have. That is a big risk, garage or street space alike. And you need to read the many posts here about insurance choices, ZTLs, IDPs, and so on. (Not arguing against a car - you need it for these plans.)