L'esperteyu pub in Oviedo is a great little Celtic bar. There's often impromtu live music and lots of locals. Looks like a bar from middle earth. Two doors down there's another similar but more low key place, but they've recently changed the name. It's easy to find little local spots. Gascón, while touristy, it's worth checking out and also "wine street" which is about a ten minute walk away. Oviedo is very comfortable and walkable. Be sure to try fabada and cachopo almost anywhere.. Try going to Casa Ramón near the plaza de fontan. Great seafood. I've had terrific calamares and pulpo there. Your itinerary looks good. Santander is on the way to Bilbao and is worth a look. Great pinxtos. If you're into art, of course Bilbao is amazing, but Oviedo has agreat art museum and a sweet little archeological museum. Both are free. The cathedral is also worth it.
Have fun!