Northern Spain encompasses a large area. If your intent is to be in the Pyrenees area then it will be mountainous with generally smaller roads with plenty of twists and turns. There are not an abundance of roads going across the mountains and most of the villages are fairly small. Driving isn’t difficult, but speeds will be slower. Along the coast, I.e., San Sebastián and Bayonne area, there are more towns and the roads will not have as many twists and turns.
The Picos de Europa park is a draw for outdoors enthusiasts and the Baztán Valley is often mentioned by the locals on the forum.
The first two weeks of September is still the main tourist season, so it might be difficult getting rooms at the last minute in popular areas.
My must see town and yours may be completely different, so pick ones that are of interest to you. Once you have a general idea of your itinerary and where you intend to stay, research the towns in those areas. Some will draw you in while you’ll be able to eliminate others. Many small towns, villages, and hamlets don’t take long to visit, but are still worth visiting.