Northern Spain

Hi,
I'm assuming your 11 days / 12 nights don't include Madrid. In 12 nights you can do, counting nights

S. Sebastian: 2
Bilbao: 3 (can do day or half-day trip to the coast, e.g San Juan de Gaztelugatxe)
Santander: 0, not that interesting (the city suffered a devastating fire in the 20th c)
Santillana del Mar area: 2, as a replacement, to visit Santillana itself and Comillas
Oviedo: 2
Santiago: 3, (can do day trip to the coast, e.g. Cabo Fisterra or Cambados)

Spare yourselves the long drive from Madrid; take the train and rent a car as you leave S. Sebastian, then drop off in Santiago.

I completely agree with balso's time suggestions. If you have fewer than 12 nights on the ground in Spain, there will be some tough decisions to make.

Unfortunately, there is not too good train connections from Madrid to San Sebastian, the route takes 5+ hrs and requires one to switch from the AVE to a regional train at Zaragoza, not a train trip I would do after a long transatlantic flight. Instead, I would recommend you fly direct from Madrid to San Sebastian, the day you get to Madrid, the flight takes only 1 hr and you will already be at the right airport in Madrid.

Then on your return from Santiago de Compostela you can just take the Alvia High Speed train to Madrid or even better just fly low cost again, if you plan on flying out of Madrid back to North America. That way you only need to rent the car from San Sebastian to Santiago de Compostela.

Also, instead of Santander and since you will have a car, I would recommend spending a few days at the medieval town of Potes, located in the Cantabrian mountains, the town makes for a great base to explore the Picos de Europa National Park. That way you add some mountains into the mix, as your current itinerary is very coast centric.

There are two direct trains per day from Madrid Chamartín station to San Sebastian, departing at 8:00 and 16:00 and taking 5.5. hours.