Itinerary Road Trip Barcelona to Southern France

I did pretty much exactly this trip 25 years ago last month. We did single night stays, but probably in less time than you will have.
This is a common debate when travel planning, to base yourself and make excursions, or to keep moving from one place to the next. In cases where you are only a single night at each place, you really don't get a chance to unpack. A fresh outfit in the evening, becomes your attire for the following day and you avoid that jumble of clothes on top of your bag that inevitably results after a stay of a few days. Continually moving is good for the hotel junkie like myself, as you get to experience the 'ins' and 'outs' of a new room every day. I think the case for 'basing' yourself increases when lots your sights are within a quite compact area, like Provence. The enjoyment of your daily excursions is not spoilt by large amounts of driving. You also get to know your chosen town pretty well, gravitating to your favourite deli and baker, and things you will remember well after you return home.

But if it was me going again this is what I would do ...
Sep 23 Fly in to Barcelona. Pick up hire car. Drive to Girona - 1 nt. (I think 3.5 hrs to Carcassone is pushing the effects of jet lag too far)
Sep 24 Drive to Nimes - 4 nts, with a stop in Montpellier.
Sep 25 Excursion to Avignon & Orange
Sep 26 Excursion to Arles, St Marie de la Mer & Aigues Mortes
Sep 27 Excursion to St Remy, Les Baux & Aix-en-Provence
Sep 28 Drive to Carcassone - 1 nt, with a stop at Pont du Gard
Sep 29 Drive to Cadaques - 1 nt
Sep 30 Drive to Barcelona - 4 nts, with a stop in Figueres

When you are driving from Spain into France, the coastal road either side of the border is stunning - N-260 in Spain and D914 in France. Leave the motorway at Figueres, climb over the mountains on the windy road to Cadaques, then north to Port de la Selva, Port Bou, Cerbere and down towards Perpignan. Narrow windy roads cling to the coast with precipitous drops to the sea below. The Monastery of Sant Pere de Rodes is well worth a stop too. Or reverse direction as may apply.

I would pick one place to stay out of Nimes, Avignon and Arles and do the rest as day trips. They are all close together. We stayed in Arles for 4 nights and it was a good base. We were not as energetic as djp_syd and went at a slower pace, but we essentially did what he/she proposed with a little more time. We went to Pont du Gard and Uzes one day, Nimes and the Camargue one day, and Les Baux and Arles the 3rd. Arles has many Roman sites and a great museum. We also stayed in Avignon 2 nights and Vaison La Romaine 2 nights (saw Orange on the way there). I think Orange is well worth seeing. If I had to skip something, I would skip Les Baux, Vaison and Uzes (I'm sure others would disagree -- and I really liked the restaurants we chose in Vaison)

We also really enjoyed Cucugnan, recommended in the France guide, near Carcassonne. We spent 2 nights there and toured the crazy castles on top of peaks! We chose this instead of Carcassonne. The roads are slow in this area as it is very hilly.

I have not visited the other places you mentioned.

Thanks to you both for your great ideas for our trip. I will read up on the places you mentioned and reconsider our first stop because of the length of the drive to Carcasonne. It is very interesting reading about all the places to see but very hard to limit where you can go in a short time. But it is certainly a lot of fun planning! Thanks again. Debbie