An off-the-beaten-path area I enjoyed (too hot in mid-summer but possibly fine by September) was the Cerdanya Valley over near Andorra. The big town is Puigcerda, with its very colorful historic area situated on a hill. There's public bus service running as far north as the old Spanish town of Llivia, which is completely surrounded by France. There are more old towns along the bus route heading south. I felt like I was the only foreigner in each of them. The train will get you to some other places, including the very pretty La Seu de Urgell (not too far from the skippable Andorra).
In addition, you can walk (about 40 minutes) or taxi just across the border from Puigcerda to Bourg-Madame, France, and take the narrow-gauge Yellow Train through the Pyrenees to Villefranche-de-Conflent. The train is tourist-oriented but an SNCF route, so the price is very reasonable. The ride is lovely, and Villefranche is a fortified (bastide) town. Totally inundated with tourists, but most of them seemingly French or Spanish. From Villefranche you can return to Bourg-Madame or go on by regular train to Perpignan, assuming you can deal with the luggage situation (not sure there's anywhere to store it while visiting Villefranche).
I don't know what transportation options (other than rental car) would get you into other parts of the mountains from Puigcerda. I suspect that would be easier from the French side of the border.
I'm not at all a beach person either, but I enjoyed a few days based in Perpignan (somewhat larger than Puigcerda and definitely not as tourist-pretty). It has a few sights of its own. My favorite side-trip was to the craft-filled town of Pezenas, but if you can deal with tourist-clogged streets in a small coastal village, Collioure is also nice in its own way. Many people adore it. It has a small modern-art museum. I also liked the less-touristy Ceret, with its own small art museum. Unfortuntately, I went to so many French art museums last summer (perhaps two dozen) that they are all muddled in my memory, and I have no idea which of those two museums I liked best.
I also stopped off in Sete and Narbonne and enjoyed walking around.
Transportation out of Perpignan can be dirt-cheap. Some buses to surrounding towns are only 1 euro, as is the occasional train ride (this is as of last year). In addition to the train option, there's bus service to Mont-Louis, which is a stop on the Yellow Train (some walking required, not signposted at the time of my visit), so that ride can be taken without trekking all the way from Barcelona to Puigcerda. Mont-Louis is fortified but looks a lot more interesting from outside the walls; it's very small. Touristy as it is, I much preferred Villefranche-de-Conflent.
Toulouse is a large city with busy sidewalks and so forth. But it's not very touristy, and the red-brick architecture is striking. I recommend spending some time there if you decide to go to Albi.
When you get up around Albi you're in Cathar country, but I don't know how to get to any of those sites without a car.
