You might look up a couple variations on the medieval pilgrimage route known as the Camino de Santiago. The classic Camino Francés enters Spain through Roncesvalles then drops down into Pamplona, Burgos, León, up into Galicia and on to Santiago de Compostela. The Camino del Norte is not as well known, and follows the coast through San Sebastián, Bilbao and Santander. Either way, the Camino was a major movement of wealth and culture along a defined pathway, in an age when most people didn’t move around very much. The result can be seen today in the concentration of medieval and renaissance architecture in even the smaller towns and villages along the Camino – churches, monasteries, hospitals to attend to the pilgrims.
Across the northern coast, you start at the French border among Basques, then move westward through Asturias, Cantabria and finally Galicia. Inland, you come to the hotter and drier regions of Navarra and the Castilian plains. Each is its own culture, with different customs and traditions, different cuisines, and sometimes a different language. Keep in mind that Spain is only slightly bigger than California, so these separate ‘nationalities’ occupy areas comparable to some of our medium size counties.
In BCN, don’t miss the Sagrada Familia, especially the forest of interior columns and Gaudi’s studio, with models showing how he used weights and pulleys to calculate the forces bearing on his arches and domes. Remember also that Spain is the country where you eat the best, so check out the La Boqueria market right off the Ramblas.
Anyway,this is quite long enough for a first post here. I have lived in Spain since 1990 and still spend part of the year in Madrid. Let me know if you have specific questions or information on particular regions!