We spent about a month is Spain -- about 20 years ago. I really liked Andalusia -- Granada, Cordoba, Seville. Toledo and Segovia were also really neat. There is a really famous palace near Madrid -- Escorial that is worth the excursion. From near Gilbrater, we took a ferry to Tangiers Morrocco for the day -- its a trip we've talked about for years!
I read the James Michener book "Iberia" while traveling. It complemented the trip well. He has a whole chapter about Santiago de Compostela -- a major medieval prigrimage site. It's a long way from Madrid and Barcelona.
The biggest "issue" we had in traveling for a whole month was that it was really hard to find laundry mats. Things have probably changed in 20 years, but they only place we could find them were at the tourist beach areas -- costa del sol and the costa brava. We went in October and the weather was pleasant, but not hot.