Madrid and Northern Spain! :)

Not too far from Santander, look for Palicio de Sonames, a restored castle that is unique in the area. Very nice. And around there are wonderful drives...the scenery will remind you of Ireland with the mountains, cattle, etc. Fascinating and well worth the time.

Santilla del Mar, between Santander and Bilbao is a must see as well. Described as one of the best preserved small villages in Spain, and near it is the Altamaria Caves (spelling?). A recreation of the adjacent cave, but extremely well done. Excellent prehistoric cave paintings.

The entire coast between Santander and Bilbao is beautiful. Be sure to drive along the sea coast highway and take your time.

Thank you so much for your recommendations!! We will definitely add those to the trip! :)

Stay 3 days in Madrid, there is so much to see and do that 2 is insufficient. Toledo is worth a day, and Salamanca and Ávila are each worth 1. Drive from Santander to San Sebastían along the coast, absolutely beautiful. The best food to be had in Spain is the Pintxo bars in San Sebastían. It's also beautiful there, right on the Playa de la Concha. I would cut Pamplona, as you could add a day trip from Santander, such as Santillana del Mar as suggested. The Altamira caves are 3km outside of the city, so yes, a car would be of benefit. Just a heads up, Santander is not Basque. It's located in the autonomous community of Cantabria, so you wouldn't have to deal with the Basque language. Northern Spain is gorgeous, I wish I could've spent more time there.

Santander is not much more than a port city. The archaeology museum and the cathedral are pretty good.

The Altamira cave has been closed for forty years.

The old coastal road between Bilbao and and San Sebastián has more impressive scenery than the Bilbao - Santander section.

I'd take Pamplona over Salamanca anytime.

If you're going into the Basgue provinces you ought to stop by Guernica.

A half day is plenty for the highlights of Avila - - you can walk most of the walls easily in an hour, the historic center is small.

Not familiar with northern Spain but I'd suggest adding a night in Madrid with a day trip to Toledo rather than spending only one night in the latter.