Northern Spain recos for a family of 6 this August

Three weeks seems like a lot (I'd love to have that long!), but I would consider carefully if you want to head both south and north of Madrid. What part of Portugal did you have in mind?
I know the northwest is a nature lover's paradise, but I have only been to Basque country. We loved hiking east from San Sebastian along the coast. To the west, there are a number of charming places. A dramatic hike is the St James way's coastal path--there are some rock formations between Zumaia and Deba. Near Guernica, you have Urdaibai biosphere reserve, which was interesting also.
You could really fill three weeks in just this region, and you can do a lot of it via train/bus, which comes in handy when you have so many that you'd need a van otherwise.

We enjoyed the rocky cost at Muxia, west of Santiago de Compostela as well as As Catedrais beach by Ribadeo. It is best seen at low tide when one can wander on the beach. Lugo has its UNESCO heritage site Roman wall that surrounds the city. It’s about a mile all the way around and an easy walk. Ponferrada has a castle. For outdoors activities there’s Picos de Europa park and the Sanctuary of Covadonga. While very touristy and crowded in the summer Santillana del Mar is worth a visit. Also along the north coast is Zumaia. It has a nice beach and up by the San Telmo chapel are walking paths that lead over to Observatorio balcón al Cantábrico. Be advised, there were some topless bathers on Zumaia’s beach. If you wanted to visit a working fishing port, from Zumaia go the three or so miles over to Getaria.

With only three weeks I’d skip Portugal especially since you want to visit Granada and Sevilla. Those three weeks will pass quicker than you think. I assumed you’d have a rental car when making my suggestions. If driving, I suggest keeping daily travel time to about three hours and making stops along the way at lesser spots to break up that time for the boys.

I think the boys, and you, would love a stop at El Peine del Viento in San Sebastian. Great fun!

Another vote for the Picos de Europa in the north. I think there are some opportunities for water activities there (canoeing or kayaking, perhaps), and if the boys haven't been on an alpine lift, Fuente De might be a hit. I don't know whether there'd be any snow to be seen in August, but I saw some in early July. I found the old stone town of Potes a charming place to wander around.

In Galicia, I liked Pontevedra, A Coruna and Betanzos in addition to Santiago de Compostela, and there are some cute small towns along the west coast.

Mixing Spain and Portugal on one trip is trickier than you would imagine, because there's very little public transportation between the two countries and picking up a car in Spain and dropping it off in Portugal would have a very high price tag. And Spain is a very large country.

Thank you! This is all so helpful! I should have clarified that we will be picking up a rental car in Madrid, and will return it to Madrid when done. I was wanting to do Madrid (briefly), northern coast of Spain, Porto, Lisbon, Algarve, then super quick stops in Seville and Granada before coming back to Madrid. At least that is my thought for now - still in the beginning stages of planning the trip. Thanks for the input!

Glad you have surmounted the challenge of finding a vehicle to fit you all, but I would scale back considerably. Definitely north or south would be a good three-week plan, and in August, I think you were on the right track to head north. Another year, come as soon as school gets out to enjoy the south when it is not 1000 degrees.

@jaimeelsabio - If we only had time for the rocky coast of Muxia or As Catedrais beach, which do you think we should try to include in our itinerary? Again, just looking for a fun place for the boys to explore, rock scramble, etc - as a nice break from all of the towns and cathedrals! :) Thanks!

Just note that the average August temperature in Seville is 96 degrees with some days hitting 104-114 degrees. The humidity averages 50 percent.
Granada is slightly less torrid with August temperature averages of 92 degrees.
You may be conditioned to 🥵 hot summers in North Carolina, but being in heat like Andalusia has in August and trying to sightsee in that heat is a whole different thing.
I would spend the entire day in the hotel swimming pool with a snorkel!
Basque Country in northern Spain has plenty to see and a lot of amazing natural beauty to enjoy. Your kids would love the fairytale castle in Olite, the “Royal Palace of the Kings of Navarre,” about 80 minutes from San Sebastián/Donostia. The caves at Zugarramurdi are also fantastic.
Have a great trip!

If you’re going in August, you’ll want to figure out the northern portion soon, or at least serious research on accommodations. I’ll be there in May-June, which is before high season (July/August) starts, but everything I read says august is mega-busy. Maybe that’s just because there a lot of smaller towns so “busy” there isn’t the same as busy in a bigger city when Europe goes on vacation in august, but something to keep in mind.

I imagine Bilbao (very interesting and much less touristy) will not be as challenging as San Sebastian in August.

There are cities where I slept in July 2016. I didn't have real problems finding reasonable rooms. (I'm cheap, but I'm also not picky beyond expecting Wi-Fi and air conditioning.)

Oviedo
Leon
Santander (not my favorite Spanish city)
Santiago de Compostela
Vigo (really not my favorite; skip it unless it's logistically essential)
Pontevedra
A Coruna

Unless you can get to As Catedrais beach at low tide when you can go down the stairs and actually walk on the beach, I’d head to Muxia. There are plenty of rocks to run around on and a small chapel there. The waves can get pretty big and they crash into the rocks making them slippery, so the boys would need to stand well back from the water in many places. Muxia can be done as a day trip from Santiago de Compostela.

The big festivals of Bilbao take place this year from Aug 17 to Aug 25, so it´ll be busy during those days. Donostia-San Sebastian celebrate its festivals the week before.

My family is currently planning a trip to Asturias (northern Spain, just east of Galicia) in July. I couple of things that I've come across that may be of interest: Jurassic Museum of Asturias & dinosaur footprints, Fuente De (gondola & hiking) in the Picos de Europa, south of Oviedo "Senda del Oso" (Bear Trail) on bikes. This will be our first time in Asturias. Having visited Galicia and Portugal, agree with others that I'd cut out the Algarve & Sevilla/Granada (scorching hot). In Galicia, Lugo is interesting, but you won't need much time there. Santiago is worthwhile and if you need a stop as you head south check out Baiona (fortress walls around the Parador). Heading inland to Portugal from there are Tui/Valenca on the border and Ponte de Lima, Porto is 1 hour south of there.

KathyW’s response reminded me that southwest of Tui are the Celtic ruins at Castro de Santa Trega. It is on a hill that provides great views of the surrounding area. Last September we went from Tui to Castro de Santa Trega before continuing along the coastal road up to Baiona.