The most popular site in Spain is Seville where all the art and crafts, festivals etc go on but that is only mostly because things like that happen in other parts in Spain.
Some examples: La Alhambra (Granada) {The most famous} Las cuevas de Altamira (Altamira Caves) (Santillana del Mar, Cantabria) Picos de Europa (Asturias, León, Cantabria) Rías Bajas (Galicia) La...