Northern Spain with 5 Adventurous Teen Boys + 2 Adults

Hi I think it's all way too rushed you only seem to be spending 1.5 nights in each location, I doubt you will remember anything from this trip other than the traveling and packaging/unpacking lol! Slow down, move less, enjoy more.

Also, I would skip Barcelona and the surrounding region this trip, you may or may not know the whole area is under serious drought and is only getting worse in the summer, best to save for another trip. Plus it's going to be hot and overflowing with tourists and pickpockets in the summer.

Instead after the Basque country I'd head west to Cantabria and Asturias, that's where the Picos de Europa National Park is. I think the Picos de Europa will be the highlight of the trip. I would spend a good 3-4 nights there. You can read more here - https://www.spain.info/en/nature/picos-europa-national-park/

I would add all of the BCN time to the Basque region. For one thing, there are drought problems in the BCN area, just something to keep an eye on, but mostly because I think outdoor pursuits need more time. (And two-night stays make me want to pull my fingernails out.)
I would also consider taking the train to Bilbao and driving around once there, but if you find good stopovers then that changes the equation.
I did not make it to Gaztelugatxeko, but I hung out in Mundaka and it was a fun area. Supposed to be good surfing at Bermeo as well. Further along the coast at Elanxtobe you can hike up as see where the whale spotters stood. Great coastal drive from Lekeitio.
The hike along the coast from Zumaia is fantastic, highly recommended! Also the hike from SS to Pasaia. The amusement park in SS was a fun diversion for two 30 somethings.
Teen boys might be interested in Pamplona. Maybe some more inland Basque region would be nice--Urkiola park?

If "more small towns and outdoor adventure" is important, take a look at the Baztan Valley, while you are near San Sebastian.
You've got small towns, waterfalls and a witches's cave. And the food is very good.

They are probably too young to like spaghetti westerns, but if they like The Good, The Bad and the Ugly, you could take them to the Sad Hill Cemetery outside of Burgos.

Thank you for the quick advice. I knew I would get the "you're doing too much" feedback! Haha...this is why my kids don't want cities after being non-stop in Japan last fall! I was hoping that the small towns would be more manageable, but I hear you on the outdoor/adventure taking more time. I will push to skip Barcelona this time (we're Californians so we get drought!!). I was there for a magical few days long ago and I was reluctant to visit after hearing how hard it has been hit by mass tourism.

Day 1-2: Arrive in Madrid on Friday mid-day, Saturday Prado/relax/recover (only 2 nights...I know it's quick)
Day 3: Noon train to Oviedo. Pick up van. Or stick to driving so we can explore more along the way?
Day 4-6: Picos de Europa and surrounding areas
Days 7-8: Bilbao (1 day: Guggenheim/wandering, 1 day Gaztelugatxeko/Beach)
Day 9-11: San Sebastián (1 day San Sebastián, 1 day surfing, 1 day Hondarribia/Passaia? Zumaia?)
Day 12: Canyoning Trip in Itxassou or Bidarray
Day 13: Best way back to Madrid??? Drive? Train?
Day 14: Fly Home to SF

I was hoping to find 3 home base areas to stay and then do trips to surrounding areas. Maybe a small town around the Picos de Europa (rural AirBnb), Bilbao (apartment in the city), San Sebastián Area (smaller beach area), plus maybe 1 night stay along the way back. Would love recommendations!

I will also consider reversing the itinerary by driving North to San Sebastián (with an overnight along the way) and ending in Oviedo with a high speed train back.

So excited! Thank you for the feedback.

I'd take the train from Madrid to Oviedo then rent the car there. In the Picos de Europa National Park, I suggest staying at either Parador de Fuente Dé or Hotel del Oso. After your time in the Basque Country you could drive down to Zaragoza drop off the car then take the high speed train back to Madrid.

Honestly I would choose either Bilbao or San Sebastian, not both, especially if you have a car.

Sad Hill cemetery was a nice visit. I will add that it is in a remote location and a couple miles down a dirt road off the N-234 if coming from Burgos. Watch for cattle along the dirt road and at the cemetery filming location.

An alternative to the filming location and along a nicer road is Cañón del Río Lobos Natural Park. A small Templar built chapel, Hermitage of San Bartolomé, is there and there are some nice walking trails. Ensure you have food and water. The canyon itself is fairly remote and a short walk from the parking area.