Split 5 nights in Provence between Avignon and Arles or between Orange and Arles

Without a car, you are better off staying 5 nights in Avignon. Avignon is really the transportation hub for the region, with good train connections to Arles, Nîmes, Orange, l'Isle-sur-la-Sorgue, and bus connections to fill in the gaps.
I see no reason to split your time between Avignon and Arles: they are a 20 min train ride from each other.
As for Orange, there is little of note besides the Theater - not the most happening town. I would not stay there.

If you are going to change towns, I'd go to Nimes, at least for the last night. It cuts an hour off your trip to Carcassonne.

While Orange is absolutely worth a stop for the theatre, the town center is super small and not all that interesting. I would not stay there. This is a total day trip destination, and I believe adequately serviced by train from Avignon. The train station is a bit of a hike from the theatre, so you may want to keep an eye for a taxi. There seems to usually be a guy hanging out at most.
Given how fast and easy it is to get from Lyon to Avignon/Arles via TGV, I would also not try to do this en route.

The Avignon vs Arles debate: I absolutely did not care for Avignon. I didn’t find it particularly fun, nice, attractive or interesting. The Papal Palace is a non-sight. There’s literally nothing inside. The bridge? You can pay an arm and a leg for admission to the lame museum and to walk out there. Again, big nothing. And it’s more a city than a town, so at times I found it a bit sketchy. My least favorite place in Europe. BUT, it is well connected. You can also easily book half-full day minivan tours out of the TI there. If you don’t have a car, these tours are really the best way to see some of these far flung sites like Pont du Garde (which is poorly served by public bus.) Two nights to take advantage of the logistics is fair. I was there for four nights and it was way, way too much.

I liked Arles a heck of a lot more. Better sights (Roman forum, amphitheater, and great little Roman museum) and much more character imo. If you can arrange to be there on market day, that’s a bonus. I’m willing to bet you could pick up similar minivan tours of Provence from there, but have no experience doing so. I’d stay the three nights here.

I stayed four nights in Arles, and found it to be a perfect place to stay. It has Roman ruins (primarily the Arena), a great history museum, charming squares, and lots of places to eat. It’s just lovely. We spent one day seeing the sites in Arles, one day visiting Avignon (which was pretty mediocre IMHO), and one day doing a minivan tour of Provence.

We took the train to Avignon for the tour, and it was a 12-hour tour, so it was a very long day. My husband thought the tour was a little too long, but I loved every second of it. We saw a lot of Provence, including Pont du Gard. It did not include Orange or Nimes. We changed trains in Nimes for our trip back to Paris and allowed enough time to see the outside of the Arena.

I know Avignon has better transportation options, but that did not offset the fact that Arles is so much nicer. At least for me.

I just got back from France! I don’t have a drivers license, so all public transportation. I also love walking. I did one night in carcassonne, then one night in Arles, one night in Vaison-La-Romaine, and one night in Avignon. The magic of Provence is in the countryside, so I liked the time in Vaison the best. (I hope it’s okay if I leave off the “La Romaine” part). From Arles it was a train ride to Orange, then a bus. Leaving vaison it was just one bus to Avignon.

I spent very little time in Avignon itself because I heard the pope’s palace was disappointing. I spent most of my day at Pont du Gard. The bus service exists, but is limited. Maybe 4 buses a day. My bus left Avignon around 11 and left Pont du Gard around 5. That is more time than needed, but it wasn’t horrible to sit and read, journal, and picnic staring at the aqueduct. I wanna note that it’s a pretty long walk from the bus stop to the aqueduct and the signage isn’t obvious. And don’t count on your bus driver being helpful, especially if your French is weak. So I was just in Avignon for dinner time and that was lovely. Connecting Nîmes to pont du gard like another poster described sounds good if you can make the bus schedule work. I didn’t go at all because I didn’t know how to connect it. I feel I saw plenty of Roman sites in Arles and Vaison.

Vaison was my favorite. I walked the 5 miles to seguret which was adorable. I loved being in the back lanes to see some farmhouses and trees and wildflowers. I hope you can find some time in the countryside. If your walking legs aren’t up for the mileage, the views out the bus window are good too.

I loved Carcassonne too. I walked around for hours. It was not too crowded on a Tuesday while school was still in session in France. (I don’t know the school calendar, but it was the last Tuesday in May).

It took me forever to find a bus schedule for Pont du Gard because it wasn’t on google maps and it’s a different municipality than the one for Vaison. It’s called Lio: https://www.lio-occitanie.fr/

And buses to Vaison-La-Romaine are on Zou:
https://itineraires-zou.maregionsud.fr/en/

Have a great trip!

Lots of good thoughts above, but I think your original plan is the best, especially with your interest in Roman ruins. I loved Arles and didn't care much for Avignon, but I was on a tour so had no need for transportation. Avignon has good bus and rail connections to places like Orange, Pont du Gard, Nimes, and Arles, so it's a good base for those places. Arles connects well to Nimes and Avignon, and is a place where you could spend a full day without going anywhere else. I also liked Vaison, which has good ruins, but I was driving at that point. You might find it "a bridge too far" with bus travel. I agree that Orange's theater is the main reason to go there (though there's also a Roman arch), and it wouldn't be a good overnight base.

Try not to miss the Pont du Gard, a near-unique Roman site with a very good museum. They offer English-language tours that go through the top of the aqueduct, though that might be another "bridge too far" for you oldsters. (I'm older than you and it took a lot out of me.) But even on your own the aqueduct is fascinating and impressive. You might look for a day tour out of Avignon that focuses on the Pont.

As for relaxed dinners, take your pick of many choices in any of these cities. I don't focus much on dinner but I'm sure there are excellent choices in Avignon, Arles, and pretty much anywhere else.

Thanks everyone for your suggestions.

After watching many videos on YouTube and reading many reviews we have decided to spend 6 nights in Nimes. We have booked a lovely apartment very close to the Maison Carrée. Only one flight of stairs and a little wrought iron balcony overlooking a lovely square.

Nimes won out over Arles or Avignon because:

• It has enough Roman sites to keep my history loving companion busy (including the Musee de la Romanite which my husband can enjoy on his own while I sit on the balcony of our apartment enjoying an aperitif)
• I liked the look of the wider streets and the boulevard feel in comparison to Arles. I am not a fan of narrow cobbled streets every day.
• It has good transport links for Pont du Gard. We will be following the bus route suggested by GeoffB and are happy to sit by a river enjoying the sunshine as suggested by squeegeeee.
• It has direct trains from Lyon

Good choice. a beautiful city. Be sure to see the Castellum, where the water that flowed over the Pont du Gard ended up to supply the city. A big cistern, with holes near the bottom to provide water for sewers and public baths, and nearer the top for watering private gardens and baths. Rich people got water when it was plentiful, the public also got water in drier seasons and years. The Romans knew social organization as well as engineering. My only complaint is that they didn't provide enough sturdy handrails for us rickety tourists! ;-)

Just a side note about Avignon, we went in 2022, on whim decided to tour the Papal Palace and my teen daughter loved it. Spent so much time inside, did not get to the included bridge tour. They have a cool ipad/tablet technology that you point it at the walls,floor,room you are in and it audio tells you about what happened in that space and the illustration of how it looked at that time in history. Not sure when they they've upgraded this tour from many commenting,but we found the Palace fascinated for that reason.
June 2024 we had to see Nimes(we could not fit it in 2022 trip) and the Nimes Arena had a nice audio tour inside that gives a lot of descriptions and history at several points throughout the Arena. Set up like "interview" of gladiator. Kept my daughter's interest. Arena was preparing for summer concert series like our American stadiums. The stage, lighting, beverage carts, etc. Amazing that the Arena venue is still in use for that purpose.