The following are my own "unfinished business" ideas for Spain, but they only are relevant if you have visited the other locations I have spotlighted.

Zamora comes recommended as a very pleasant, very off the beaten track town with a good parador.  (A Spanish friend warned me, however, that it is a bit austere, until I reminded him that I am among the minority that prefer northern Spain to southern Spain.)

I should visit Extremadura.  Cáceres is the most famous destination there, but I understand that Trujillo is very memorable and maybe even a better place to stay. Very dusty but good things to see and a great atmosphere - the most "old world" place these American friends have seen in Europe, after living in London for more than 20 years. (Similar was Guadalupe.)  One possiblity is to dust off this driving itinerary:

Extremadura (Spain) and Alentejo (Portugal)

Cuenca is said to be a beautiful, picturesque town, built on cliffsides.  (I understand that the bus leaves hourly from Madrid (Auto Res, Fernandez Shaw 1.))

Aranjuez (the palace especially) could be a daytrip from Madrid or combined with a visit to Cuenca.

Santander is a beautiful fishing village on the Bay of Biscay.

Alcala de Henares is a world heritiage site, the home of Cervantes, and near Madrid.   Alcalá de Henares

Ronda comes highly recommended both by Spanish friends and by an American family that went there.  It is an old town, perched in the mountains above the Costa del Sol.  Stay at the parador.  http://www.parador.es/ingles/85clicks/rond/rondpaning.htm

I really should go back to see the Alhambra with my wife; I haven't been to Granada since 1972.

I have been to Toledo three times, but feel that I barely have scratched the surface.  Next time I would love to spend the night within the walls of the city and have the opportunity to wander the city during first light.  Personal To-do List for Toledo

Finally, my main unfinished piece of business is to walk the Camino de Santiago de Compostela as a pilgrim.

Among the resources to use are this site for Romanesque and Medieval art:  http://www.arteguias.com/index.htm