We are a family of four and wondered what would be the best way to see Orvieto and Civita as a daytrip from Rome...public transport or rent a car for the day?
Take the train from Rome to Orvieto. From Orvieto you have two choices: rent a car or take the bus (in 2006 about 6/day, 1 hr each way, buses don't operate on Sunday).
We took the train from Termini; very easy.
The least expensive round trip tickets for 4 to Orvieto (2nd class regional train) would cost EUR 56.80. Add to that 4 round trip bus tickets, and it probably makes better sense to rent a car for the day and drive from Rome. Even if the cost was a bit more, you'd make up for it in convenience.
Take the train. The most inconvenient thing you could do would be to rent a car. Horrid driving into and out of Rome. Would take much longer to drive than to take a train.
julie, a day trip from Rome to Orvieto is fairly easy, as there are good train connections. When you arrive, buy a ticket at the Tabacchi in the station (good for 70 minutes) for the short Funicolare ride up to the town).
However, getting to Civita is a bit more complicated. You can certainly use the blue Cotral Buses for the trip (as I recall, about an hour as mentioned in the previous post - again, buy tickets at a Tabacchi in Orvieto). However, when you arrive in Bagnoregio, you'll be using a small shuttle Bus that departs from the main Piazza. The return journey from Civita to Bagnoregio is sometimes a bit "erratic" as the shuttle driver sometimes takes a "siesta" in the afternoon (the day I was there, it started raining and I got soaked waiting for the shuttle - thankfully a kindly couple from Milano gave me a ride back to Orvieto).
Your trip is certainly possible to do without a car, but you'd have to plan very carefully to make sure you were back in Orvieto to catch the last train to Rome. Rick's Italy 2008 guidebook has lots of information on the Bus and train schedules. This would be a lot easier if you planned at least a one-night stay in Orvieto.
When you do get to Civita, you might consider lunch at Trattoria Antico Forno. I really enjoyed the food, and my visit with Franco (the owner) was one of the most memorable parts of my trip.