Asturias Itinerary

I have not been to Asturias but considered it in some recent planning, so I'll just share some things I was pondering in hope it will help:

1) Determine how far you can drive on arrival after an overnight flight (if that is your situation). For me (my driving partner rather), it is just over two hours. So, definitely compare getting the car at the airport and stopping for a night versus taking a train (which is good napping time in my experience). I am not seeing much of a difference in either renting from the airport or renting elsewhere, so hopefully the price hikes in other European countries are not affecting the Spanish rental car market.
2) You will need to be in Madrid or nearby the night before your outbound flight, so take that into consideration--I am not 100% sure what your timeline is. But I do think if you start looking at Basque country, you could very well end up with a Basque country trip--maybe clarify the timeline. You can easily spend all the time in Asturias.
3) I was looking at pairing Oviedo with a coastal town like Lastres or Cudillero. Anyway, you are spoiled for choice. I could not make it come together this time but hope to get there eventually. good luck!

Oviedo has some very, very old churches and probably is to be preferred from the sightseeing point of view. It is pretty quiet for a Spanish town of its size (pop. 220,000). Its Wikipedia entry lists quite a few ancient churches as well as other buildings of importance and some museums.

I read somewhere that Gijon is where folks from Oviedo go for excitement. Gijon is an industrial town of about 270,000 with a small historic center. It also has some early-20C architecture. It's Wikipedia entry includes a long list of museums and notes that tourism is becoming increasingly important.

Your path from Bilbao to Oviedo or Gijon will take you near the Picos de Europa National Park. It has lovely scenery, an Alpine lift (at Fuente De) and the lovely (not undiscovered) old stone town of Potes. The area seems to be a popular location for outdoor activities.

I'm not a beach person, so all I can suggest on that topic is that you put together a list of nice-sounding beaches so you know what is nearby when you have a nice day. That part of Spain tends to be cool and rainy. When it's not raining it's often overcast. Best to be flexible about which days will be stop-at-a-beach days.

Interesting choice of regions as it isn’t very large. We visited northern Spain in 2017 and loved Galicia, Asturias, and Cantabria. Upon arrival we picked up the rental car and headed to Zamora. En route we stopped in Arévalo for an early lunch and to walk around. There is a castle in the town, but it was closed, so we just walked around it. After leaving Arévalo we drove directly to Zamora for 1 night. Zamora is about a 2.5 hours drive from Barajas airport if driving directly to it. From there we drove to León (2 nights), stopping en route in Valencia de Don Juan to walk around its castle. Skipping our Santiago de Compostela portion of the trip, I’ll pick where we stopped at Cathedral Beach/As Catedrais beach, just west of Ribadeo. If possible get there at low tide so you can walk on the beach beneath the arched rocks. There are a number of beaches along this stretch of Spain. Just outside Oviedo are a few old churches; Iglesia de San Julián de los Prados - Santullano, Iglesia de Santa María del Naranco (built in 842), and Church of San Miguel de Lillo. We haven’t been to the churches, but they’re on our to visit list. On our trip we then visited Cangas de Onis and walked around it for a while before heading up to Sanctuary of Covadonga in the Picos de Europa park. From there we went to Santillana Del Mar in Cantabria for 2 nights. Santillana is full of tourists during the day, but empty at night. From there our trip continued east. In Cantabria, south of Santander is another church, Colegiata de San Martín de Elines. Nearby it is Cueva del Agua in Orbaneja del Castillo. Returning to Madrid you could stop in Burgos. I haven’t visited Gijon or Bilbao recently so can’t comment on them.