Northern Spain

I liked Oviedo and Burgos. Both have attractive historic districts and lovely cathedrals.

Oviedo has some extremely early (9th century?) churches; your car will be helpful in reaching one of them, but choose a hotel with nearby parking.

Burgos gets a lot of pilgrim traffic and is built along an attractive river. It's a bit livelier than Oviedo and has a couple of very interesting convents (car helpful again). As mentioned in the RS guide book, Burgos is an exceptionally windy city. As with Oviedo, consider the parking situation when you choose a hotel.

San Sebastian is not large. If you have extra time there, consider visiting the coastal town of Hondarribia over near the French border and/or one or more of Zumaia, Zarautz and Getaria between SS and Bilbao. Lekeitio is reportedly very picturesque, but I have not been there.

You might try to visit Laguardia while you're in the La Rioja area. It's a very atmospheric hill town between Vitoria-Gasteiz and Logrono (closer to the latter).

In addition to its medieval district, I enjoyed the early 20th century architecture in what I call the "downtown area" of Bilbao.

rary:
Santiago de Compostela - 3 nights
Via Lugo & Ponferrada to Leon - 2 nights (Astorga is worth a stop too)
Oviedo - 2 nights
Via Asturian coast to Cosgaya/Picos de Europa - 2 nights
Via Santillana del Mar to Bilbao - 2 nights
San Sebastian - 3 nights
Via Pamplona to La Rioja - 2 nights

Before you decide, you should check to see if there are fiestas going on in the cities you are going to visit. For example st. Mathew, or San Mateo is the 21 of Sept. I know that in both Oviedo and Logroño it is fiesta for example. Some people like them some people don't. Either way booking a room can be difficult.

Thanks for the itinerary suggestions and festivals date! I think I will add two nights along the coast, possibly near Colunga or Ribadesella. Any suggestions for location or hotels?

Since your adding a couple nights I really enjoyed Burgos. Along the northern coast I love Santillana del Mar. Santillana is like walking back in time. Personally, I see nothing special about Bilbao or San Sebastian. There are so many lovely small towns along or near the route you're taking; Penferrada, Santo Domingo de la Calzada, Alba de Torres, Avila. Near San Sebastian is Hondarribia, a small town with a nice historic center. When planning my trips, once I have a general idea of where I want to go, I'll look up many of the towns on TripAdvisor to see what each town has to offer. I frequently find place I otherwise wouldn't have known existed.

I haven't been to Colunga. I thought Ribadesella was pleasant enough for a couple of hours, but I can't imagine spending two days there. Santillana del Mar is much more distinctive, though certainly touristy. And it's quite small; two days would be at least one too many for me. The nearby Comillas has a couple of interesting modernista buildings that can be toured, and there are caves near Santillana del Mar with prehistoric art; Google for info on advance booking.

I, too, liked the several Basque Country coastal towns I visited, but I'd do all of them as day-trips from Bilbao or San Sebastian: Hondarribia (east of SS, the largest and needing the most time), Zarautz, Getaria, Zumaia, and Bermeo (east of Bilbao).

Unlike the other poster, I enjoyed both the large cities, preferring Bilbao (be sure to seek out the medieval district, which is not near the Guggenheim). Foodies and bar-hoppers may especially appreciate the restaurant district in San Sebastian.

Keeping in mind that it can be cool and overcast/rainy in the area you're visiting even in summer, I don't know that I'd want to commit to too many days staying in small coastal towns where there's not much to do indoors. Vitoria-Gasteiz is worth some time and has indoor sights to visit. Obviously, V-G would come at a different point in your trip from a place like Oviedo or Burgos.