First recommendation, listen/watch a bunch of Rick Steves stuff. He does a great job of teaching you how to tour more like a local than a tourist. Also, listened to podcasts, etc. to learn some of Spanish history and stuff. It really made the whole experience more understandable and enjoyable.
- Madrid was really neat. We hit up the royal palace; very impressive. Also attended a bullfight and got upgraded tix to sit closer to the action. It was so fascinating/brutal. Also, found a Goupon (yes, Spain has Groupon) to attend a flamenco dance in a small venue. Incredible. Also went to the Parque Retiro.
- Took a train for a day visit to Toledo, the "Spiritual capital" of Spain. Really neat city, cathedrals, etc.
- Rented a car and stopped by the Valle de los Caidos. Worth a visit.
- Stopped by Segovia to see the ancient Roman aqueduct. My engineer brain loved it.
- San Sebastian...I can't say enough about this place. It may be the most beautiful place on the planet.
- Stopped by Montserrat on the way to Barcelona. Wow.
- Barcelona was very enjoyable too. Definitely more touristy. Visiting the Sagrada Familia was a sacred experience. So unique and inspired.

Happy travels!