A little more info here would be great about interests..culture..natural beauty..food..beaches..mountains??? the whole melting pot?
If you want to rent a car and do what the locals do...then read on..
If you like hiking and getting off the beaten track would definitely recommend the Parque Nacional de Somiedo..be sure to take a tent to camp up here too...
Also the northern beaches in Asturias/Galicia are just brilliant..very local..and food superb..
driving through Navarra and Aragon and the little towns are just glorious..
Pyrenees..so many options here - we did the estany de sant maurici and just beautiful..
the Catalan coast is also idyllic - full of little turquoise coves - but be sure to research this well..
I love Andalucia but getting out into these other parts of Spain was just incredible - think about the main types of things you want to do and then plan out just a few key regions...
Enjoy,