Late March on a Sunday, Assisi or Civita

Assisi/ check that you can tour all levels of the Basilica on a Sunday. It is filled with beautiful frescoes.

There's a lot more to see in Assisi. Civita di Bagnoregio is interesting enough if you can manage to see it efficiently, and you're going to have a car. Could you see Civita on the day you arrive in Orvieto or the day you depart?

Or try for a really early start and see whether you need the full day in Assisi. I loved walking around the upper (historic but non-touristy) part of town. It's only the area near the basilica that is awash in tacky tourist shops. I saw very few tourists in that uphill area, so I think most folks don't spend as much time in Assisi as I did, which is why I am suggesting that you might possibly be able to stop in Civita on the way back to Orvieto.

Civita is a curiosity that IMHO should be well down the list of must sees. It is an interesting site. It is most amazing from a distance. Actually within the walls it is just another old Italian town with pretty much nothing to offer except crumbling edges. Here was our visit:
https://janettravels.wordpress.com/2011/10/07/civita-de-bagnoreggio-adrift-in-the-clouds/. Worth a quick stop on your way from here to there, but I wouldn't make it a destination.

Assisi on the other hand is fulled with interesting and important things. We stayed at the inn run by English speaking nuns right in the center of town -- wonderful place with lovely views and close to the Cathedral and to Santa Chiara and a short walk to the Basilica of St. Francis. We went for vespers at San Damiano the church where Francis received his call to serve the Lord. It is a very evocative cave like old church just below the town. We saw it before the earthquake so I don't know how the frescoes have faired, but those by Cimbabue and Giotto in the Basilica are a wonder. This is just a more important and significant sight.

This is part of a 14 night on the ground reprise of central Italy (we have been 3 times in past 19 years, excepting for Rome we are looking at lesser sights at a slower pace). have four nights in Rome (flying into Roma), last there in 1999. Hope the March crowds will be small (er) and the weather tolerable. We have been to Venice, CT, Milan, The Lakes, Florence, Naples, Pompei, and the Amalfi coast, pretty much exhausted Rick's books (and ourselves at times!).

Have three nights in Orvieto, planning mid morning arrival in Orvieto and see most if not all of it that day. Considering second day at Civita, perhaps circle up to Pienza or Monte-Pulicianio - we did the Tuscany thing (!) from an agritisumo about 10 years ago. Third day thinking Assisi and then if time permits freestyle a little, out early the next morning to Bologna.

Next three nights In Bologna (I need a book quickly Rick), follow with three nights in Padua(hotel)/Vicenza/Verona and then train to Milan (also been there) and overnight (#15) for the flight home.

I agree with Janet about Civita. I'm so not a fan. Not only does it look better from a distance but if you have anyone who is sensitive to heights what you can't really tell from the pictures is that the causeway going over to it is high! Once you get across there is literally nothing there - very few are allowed to live there. I think our Rick Steves guide told us 5 people were living there full time when I visited several years ago. There are a few places to eat. One church. Lots of cats. Only tourist visitors.

+++ to Assisi!

John, as others have said, Assisi is MUCH larger and MUCH more interesting than Civita. Assisi has various churches, beyond the principal Basilica, and has sites beyond the town also. I remember there was even a Nepalese or Himalayan Museum on the main street of Assisi, put together by a group doing development work in that part of the world, not sure if that is still active, but it interested me. Also some nice restaurants and shops in Assisi. You will be hard pressed to cover most of the sites and experiences in and around Assisi in just one day.
Civita, on the other hand, is a small relic, now just a tourist site. The last native of the town died a few years ago. The pedestrian walkway is long and quite sloped up when approaching the town. Not much there, so does not need much time to explore, although a significant investment of time to park and then walk the distance to and from. I visited on a Sunday afternoon in October, and both the parking area near the walkway and the town of Civita itself were very crowded with Italian tourists from Rome, a nice weekend excursion away from the busy city, I imagine. Not sure if you would find the same thing in March, but maybe if it’s a nice day.