San Sebastian and Bilbao. The entire drive should be beautiful. I spent six months in downtown La Coruna and 6 months in downtown Bilbao. In Coruna, you should see the Torre de Hercules, the fortress from which the Armada sailed and walk around downtown.
In Bilbao, I actually like the Bilbao art museum more than the Guggenheim (although the Guggenheim is a really cool building). Bilbao has a great metro and a historic downtown area.
You should definitely stop in Santiago de Compostela to see the cathedral.