1 Day, 1 Week, 2 Weeks, 1 Month ...
You cant not miss it
Euskadi, Asturias or Galicia (visit the three is a good idea)
Cantabria, if you like touristic place in the north sound good too.
Barcelona, Madrid, Sevilla (for visit important cities)
Toledo, Navarra, Salamanca
Valencia, Girona, Málaga (for important touristic points)
A diferent idea for know Spain
A lot of people use to know Barcelona, Madrid, and south side... Well, is no a bad trip, but, you miss some place... I think if you are really interested in know Spain no touristic place, just know the diferent Spains ;-). Try to visit, one or more region in north of Spain... Galicia very interesting place with one religion tradition (see compostela) celtic culture, Asturias minner region, the most espectacular views you will find in Asturias, and very quiet areas mixed with really happy areas, celtic culture too. In spain capitol of hospitality people (but look in the small towns, is really nice) Cantabia In the north too, good with views, and filled with touristic places. Euskady or Pais Vasco: Really nice people, really with views, and a complete diferent culture.
If is posible may be great to know Galicia, Asturias and Pais vasco.
You must too know Salamanca, a study city, with very famouse nighlife, and of course one of the most importants culture cultures point in Spain.
If you visit Madrid, please, dont forget the Park In sunday, with a lot of drummers improvisations in the side of a lake..., and you must grow at the same time you listen this improvisation, or just go to the point in view it, is nice... visit the album about Madrid in gallery.
Is there a lot of interesting points to visit, may be great if my Spanish companions help me and put here more tips!