That's a great idea! As a matter of fact FEVE runs an all-inclusive (like a cruise - but very expensive) 8-or-10 day excursion from Bilbao to Santiago (the last part by bus). The scenery along that route is awesome! I would suggest Limpias for a stop if they have completed the new Parador there. If it were me, I would either start my trip or make a prolonged stop in Santander. From Santander's bus station (across the street from the FEVE station) you can catch a bus to Santillana del mar. Or you can daytrip from Santander to Comillas, Santillana del mar, Liebana y los Picos de Europa, etc. I day-tripped on FEVE to Limpias from Santander (came back via Laredo by bus) and to all those other places.