I agree with Tincatikner, the FEVE is fun and the route is beautiful. The FEVE is a narrow gage railway and includes a route that runs all along the northern coast from San Sebastian to Ferrol.
I also agree with stops in Santander, Llanes, and Oviedo. You could also stop in Ribadesella to visit Tito Bustillo Cave, which was a highlight of my visit last year. They recommend that you get tickets in advance. The tours are only in Spanish but there a also a wonderful museum that explains everything in English.
https://centrotitobustillo.sacatuentrada.es/es. The website says they're closed Mondays and Tuesdays.
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The Tito Bustillo Cave is one of the world´s collections of Palaeolithic rock art. In July 2008 it was declared, together with four other Asturian caves and twelve more in Cantabria and the Basque Country, a World Heritage Site due to its extraordinary examples of Palaeolithic rock art."
Enjoy!