we spent 10 days there just riding around picos de europa.
here's some routes.
along the coast freeway to laredo.turn right on the n629 towards vilasante.fantastic mountain pass.
turn right c6318. all excellent smooth fast roads.head west n232 / ca171 then left up the n611 to aguilar de campoo.turn right there and just about any road is brilliant.definately recommend potes to reiano..amazing pass.
reiano to cangas..stunning.
get a michelin map and just trawl the roads around that area. great roads, mostly empty, cheap nice places to stay...we had a blast. you will too.
and santander is such a better place than bilbao!
have a good time..we're jealous..:-)