I'm also one of those pilgrims who cannot not stop or detour to admire architecture, culture, nature... (we here call such 'inability' to distance oneself from a beloved interest/proffesion ''poklicna deformacija'' = professional deformation - being a historian and an archeologist, you can understand my 'problem'
).
I particulary like exploring little historic towns - santillana del mar springs to mind first, but there were also irún, hondarribia, orio, getaria, castro urdiales, laredo, llanes, villaviciosa... big towns always offer (too) many choices for a rest day. I particulary liked bilbao where I stopped for an extra day, and donostia.
I toured several museums. in donostia san telmo museoa with its collection of extraordinary basque women headdresses and giant processional statues. in bilbao I loved euskal museoa bilbao (the basque museum) with a beautifully carved medieval cross (with santiago), a prehistoric idol, a closet with men in scallop skirts, and the relief of the whole basque country. in santander I was impressed by the archaeological museum with informative displays and giant round prehistoric (funerary?) stelae.
I missed both caves with paintings - altamira was closed on monday (grrr) and in ribadesella I was too early for tito bustillo.
then there were the churches, their architecture, their portals and their reliefs. I am very fond of medieval reliefs in all sorts of guises, the older the better (read: romanesque, preromanesque and gothic). to mind spring: the vast porch of the zenarruza monastery with its carved eaves and an alquerque game carved in the stone bench, the cloister of santa juliana in santillana del mar with its capital thought to represent exorcism and another capital with a knight returning home. the painted portal of santa maria in deba with the chorus of also female musicians, a lovely representation of nativity, and a fine portait of jesus(?). I loved santa maria del bareyo with it stunningly preserved and very unusual decoration, including a pillar-statue in the apside, grotesques, and a long-horned bovide. the portal of santa maria de la oliva in villaviciosa has a lovely portal with a very fine carved boar hunt and san juan de amandi an unusual round porch. also the huge monoliths in san miguel de arretxinaga at the start of markina. if you can, make a detour via valdediós - preromanesque s salvador is amazing.
too many ideas?