Traveling to Northern Spain

You can easily take a train from Zaragoza to Bilbao and pick up a car when you leave...minimizing parking and city driving. The coast all the way to Galicia is mouthwatering. We stayed 3 nites in Ribadesella and 3 nites in Ribadeo. All the little villages along the coast were lovely. The driving was great, very little traffic and excellent roads, no tolls!
Tarazona from Zaragoza and Castro Urdiales from Bilbao are both worthy day trips.
Buen Viage Brad

It sounds like a great itinerary. We've being doing similar by train -- trip reports here and here. You might consider visiting Pamplona between Zaragoza and Bilbao as it definitely checks all your boxes. Also this stretch of Northern Spain has a new marketing brand of "Green Spain" -- because it is lush and beautiful. But it also means it rains. A lot! Having a car will help with that, but just know it is a strong possibility. We're in Zaragoza now. You'll love it!

With four nights between Zaragoza and Bilbao, consider stopping in Olite (Royal Palace), Artajona (old town walls), Estella, Irache (to see the wine fountain on the Camino), Monastery of Iranzu, Laguardia, and Puente la Reina (bridge). Tarazona, as already mentioned, is worth a short stop to walk around the castle and look at the plaques on the houses to see who was voted witch of the year. It is the only town excommunication by a Pope for witchcraft. Driving in town is for residents only. There is a parking lot at the town entrance. If you’re religious then Monasterio de la Inmaculada Concepción in Ágreda is worth a brief stop to see the Blue Nun who was supposedly able to bi-locate (be in two places at once). For an impressive view, stop in San Vicente de la Sonsierra and walk up to Mirador San Vicente De Sonsierra by the castle and Ermita de San Juan de Arriba. The chapel was closed when we were there, but one can walk to the top of the tower in the castle ruin. There is no parking at the castle. We parking in a lot by Colegio Público Entreviñas, had lunch in town, then walked up to the castle.

From the national park, Cangas de Onis is worth walking around as is a stop at As Catedrais beach near Ribadeo. It’s best seen at low tide. If you have time Lugo is a nice stop to walk around its Roman wall that is a UNESCO World Heritage Site. Closer in towards Santiago de Compostela is the small Igrexa de San Salvador de Vilar de Donas. The original 15th century paintings behind the alter make the stop worthwhile. Just outside of Santiago is Monte de Gozo to see the Pilgrims statue. There is plenty of parking. Since you will be in Santiago for a few days, drive over to Muxia on the coast. The town is nothing special, so go towards the lighthouse and park at the lot by the Nosa Señora de la Barca chapel. The rocky coast is beautiful.

Try to book a room in the historic Parador
Santiago de Compostela. It overlooks the main plaza in front of the cathedral. Historic, built by Isabella and Ferdinand in 1499 as a hospital. You can watch the pilgrims arriving in the plaza at the end of their long journeys.

Have wonderful seafood in SdC. I miss much of Green Spain as it's not as crowded and hectic as Barcelona or southern Spain.

As a Bilbao local, my recommendations for day trips: Butron-San Juan de Gaztelugatxe-Mundaka-Bermeo-Gernika and the Assembly House. Also, Puente Bizkaia and the old fishing port of Algorta (Puerto Viejo). A day trip to Laguardia in the Rioja Alavesa wine region (one hour drive only), and visit some family owned wineries. Getaria and Hondarribia on your day to Donostia-San Sebastian. And if you want to go off the beaten path, Elorrio-tombs of Argiñeta-Loyola-Arantzazu. On the Rioja day, a visit to our capital, Vitoria-Gasteiz, is also a really good idea.