I didn't have a car, so my experience is a lot more limited than balso's. I do agree about the skippability of Santander (not a bad place, but mostly less than 100 years old due to a massive fire). I can add these as possible stops:
- Leon: Attractive historic center with lots of lively restaurants and bars, plus one building designed by Gaudi.
- Astorga: Some modernista architecture
- Betanzos: Nice not-too-large city in Galicia, not far from A Coruna (which I also liked a lot). Didn't seem to have any foreign visitors aside from me.
Also in Galicia but farther west:
- Santiago de Compostela: You don't have to be religious to find SdC lovely, though you must expect a great many pilgrims.
- Combarro: Cute but touristy small town on the west coast.
- Cambados: Attractive small town on the west coast with few, if any, foreign tourists.
- Tui: Border town with cute historic center, just north of the Minho River.
I think the very old churches are the major reason to go to Oviedo. It's not tiny, but it's a quiet city. I've read that the locals go to the industrial/port city of Gijon for excitement. I day tripped to the latter and found some attractive early-20th-century buildings; there didn't seem to be more than a small area of older architecture.