Just follow the coast road from Santander. There’s a motorway parallel, so most of the traffic is on that.
You’ll go thru, Laredo and Castro Urdiales both nice towns (and a few others)
When you get to Zierbena, there’s a bike lane that takes you to Portugalete, take the bridge over and then follow quiet roads to Gernika (via Mungia) . (This avoids Bilbao). From Gernika the coast road is spectqcular pretty much all the way to San Sebastian. You go thru some lovely villages like Lekeitio, Ea, Zarautz, Getaria.
If you need any help/guide for a day, let me know. I live near Bilbao.