Orvieto - few hrs

2.5 hours is probably enough. Orvieto is not very big.
Visit the Duomo, maybe walk out to edge to see the views, find a quick lunch. There are a couple cafes/restaurants right on the Duomo piazza and some just down the few streets from there.

One question- will they be dropping you off IN town- top of the hill? Or will you need to take funicular/bus to get up there?
That could easily take 30 min.

I did not find the Sacro Bosco to be compellingly interesting. Given that meals tend to be leisurely in Italy, much of the time you have allotted to Orvieto will be spent in a restaurant. Orvieto has one of the most magnificent cathedrals in Italy, a must-see. There is also a small museum with Etruscan finds nearby. A couple of other attractions, of which I have no personal experience are Patrick’s Well and the underground tour. Just wandering the streets in the historic center is appealing. No need to add another destination.

I would want to have more than 2.5 hours in Orvieto if lunch was also part of the plan. The Duomo is magnificent and the town is charming, and there are a lot of nice ceramic shops too.

We did something similar: hiring a driver to take us from Rome to Orvieto with stops at Sacro Bosco and Civita di Banoregio. That was one of our favourite days in Italy.

Sacro Bosco was nearly empty on a November, weekday morning, and, as it was a sunny day, we really enjoyed the fresh air and birdsong after being in Rome. It made for a lovely break. The ruins are a lot of fun. Some people sneer at Sacro Bosco, because the structures aren't "real" ruins, but we liked them and took some fun photos. An hour is a decent amount of time there.

In Orvieto, we had lunch on a patio overlooking the cathedral and went in afterward. You would most certainly have time for that. If you want to do the underground tour, however, you might not have enough time, and you'll want to check the schedule for that to see if it would be workable with your schedule. http://orvietounderground.it/index.php/en/

Orvieto is a rich destination, but suitable for a half-day when in transit. But it is naive to plan on a real lunch while a paid driver is waiting for you. You need to have a pre-made, to-go (and my own ironica typeface, probably unrefrigerated in the display case!) sandwich while walking around. This is the native land of "slow food" and you would be crazy (even for a Michelin restaurant) to cut into a minimal 2.5 hour stop.

Actually, this is very doable. There is a parking lot very close to the train station. Your driver can wait for you there. Take the funicular up the hill and work your way to the Domo and wander the nearby streets. If you think the underground is worth doing, go for it. You'll get to see where they pressed olive oil. raised pigeons...get a few views that you can also get without the tour.
To eat, there is a very quick and consistently good small restaurant next to the funicular entrance. They serve the local business people. Tourists go to the larger one to the South. Order a carbonara and expect to be in and out quickly after enjoying a delicious and inexpensive meal.
If a sandwich is your thing, across the street at the train station...you can get one there, or that touristy place already mentioned (which actually does have very good and slow food).

I think 2-1/2 hours is a painfully short time in Orvieto. The funicolare runs every 10 minutes (every 15 minutes on Sunday). I don't remember how long it takes--I'd guess between 5 and 10 minutes. You'll need to buy tickets, too. Most of the key local sights are near the western end of the upper town. The funicolare is at the eastern end. Google estimates the walking time from the top of the funicolare to the Duomo at a minimum of 10 minutes. All told, it will take close to 30 minutes to reach the Duomo from the bottom of the funicolare, so that's a round trip of about an hour before you've actually done anything. I'd find that frustratingly rushed even if I didn't have to allow time to grab something for lunch.

Having just been there recently, 2 1/2 hours is really cutting it short. The fastest way up the hill is by cab - but they don't linger around the station. We had to call. The # is on a little hut next to the station. We originally planned on only a couple of hours, and with all the feedback we received, we cut out a day in Rome and spend the night. We were glad we did. It is quite a lovely town, laid back, and a great place to unwind. We did the underground tour, the Duomo, had lunch, walked the town and browsed the shops, and enjoyed it immensely. Any extra time you can add, do it.

Wow! Thanks for all the wonderful feedbk!
We have added an hour to Orvieto - so we will have 3 1/2 hous there. Your agenda ideas and
food suggestions are very helpful!!
Thanks again!!

"Take the funicular up the hill and work your way to the Domo and wander the nearby streets."

No.

The funicular goes up to Piazza Cahen, one KM downhill from the Piazza del Duomo. Instead take Bus 1 from the station to the Piazza della Repubblica. End the visit at the Duomo and walk down to the funicular.

They have a car and a driver. If anything, they should drive around to the back parking lot and walk from there.
But good grief, it takes about 10 minutes to walk a click. Take the funicular.