Lots of food for thought on this thread. Just to add a bit more ...

If your husband's interest is primarily Gothic architecture, I think you'll find that the richest pickings are in northern France. You could stay in Paris for two weeks and make several day trips to see the cathedrals mentioned by twoflower, above. (I'd add Reims to that list, and others have mentioned Chartres.) Or you could spend a week in Paris and then make a week's driving tour of the northern cathedral cities.

But there's more: if his interests extend to Romanesque architecture, there is a gold mine around Clermont-Ferrand, right in the middle of France. (You can see eight church photos on this page, all taken within a 50-mile radius of Clermont-Ferrand; just click on any thumbnail to see it enlarged: www.pbase.com/anselmadorne/france&page=4)
It's a beautiful area that I think is often overlooked.

As others have mentioned, there is probably less interesting church architecture in Provence, but it is rich in Roman ruins. If you were to base yourself in the Luberon, as mentioned by Stu, above, you could easily visit Vaison-la-Romaine, Orange, St-Rémy-de-Provence, and Arles. If you stayed in the St-Rémy area, you could also push a bit westwards and see the Roman ruins in Nîmes and Pont-du-Gard. Provence in general and the Luberon in particular meet your interest in "clusters of small towns."

There are other architectural gems in Provence: in Arles, the Église St-Trophime and its accompanying cloître are fascinating. The facade of the abbey church in St-Gilles is said to be one of the finest examples of Romanesque sculpture in the south of France. And finally, there are two Cistercian abbeys that are breathtaking: the Abbaye de Sénanque and the Abbaye de Silvacane. Both are easily accessible from the Luberon. (Cistercian architecture is unique, sort of the Shaker furniture of that era: simple, unadorned, beautifully proportioned, tranquilizing, and humbling.

At the risk of recommending any research to an architect, there are two books that have helped me immeasurably: "Early Medieval Architecture" by Roger Stalley and "Medieval Architecture" by Nicola Coldstream, both published by the Oxford University Press. The texts are easy and the drawings and photographs are excellent.

Anselm

Hmmmm ... it looks like only part of that link is "live". If you click on it, you'll need to go to page 4 to see the photos of the Romanesque churches.

AA

Hey ShariB,
Various posters have mentioned that lavender fields are blooming in June and July in Provence-photos of same look so wonderful.
We saw Arles and Avignon (in March as day trips from Nice) and, in terms of visual impact, loved Arles and were indifferent to Avignon. Others will heartily disagree! We are staying in Aix this September so I'll report back about that city as a base.
Check other posts for ideas about spending the night in places easily accessible to airport when you leave. OR as OP suggested fly out of city in the south.
And yes, "everybody" speaks English these days but having some French is very much appreciated by the French. Also, always say hello before anything else. It is considered tres rude not to.