The Basque Country and Asturias
San Sebastián and Bilbao, both in the Basque Country. Asturias beyond it. Three days at the shortest, eight at the longest. Departures that shift to fit you, not a fixed calendar. Each city carries its own specialist, and the knowledge doesn't reset when you cross into the next one — it builds with you.
The North doesn't announce itself. It rewards the traveler who comes to pay attention.