Barcelona -> San Sebastián -> Madrid for 17 days in October. Unsure how long to spend in each place.

3, maybe 4 days in San Sebastian is enough. By October spending time on the beach not a big draw, there are some things to do, leaving food (which is great). Then the balance in Madrid and Barcelona, I would probably leave more days for Madrid than Barcelona.

How do you plan to travel between the cities? Either by car or train, you will have some long travel days.

If you are going to day trip to Bilbao, I'd want to have three full days in San Sebastian. Another day if you'd like to visit Hondarribia, a beautiful seaside town close to San Sebastian with a castle of Charles V. San Sebastian has an excellent Basque museum.

In terms of Madrid and Barcelona, I'd watch some Rick Steves episodes and look at the site seeing opportunities in each city. Also look at possible day trip options from each. For Madrid, Segovia and Toledo come to mind. For Barcelona, Montserrate, Girona, and Figueres are possibilities. For me, personally, we were in Barcelona/Girona, about eight days and could have stayed more. It is a very unique city. I thought Madrid was very pretty and lively, but I think there are so many more historic and interesting cities in Spain than Madrid. They do have a lively food scene (I think all Spanish cities have great food, however), wonderful art museums and a lovely cathedral.

I´d spend at least two days in Bilbao (great urban planning, architecture, food, old quarter, Hanging Bridge, funicular, Azkuna Zentroa...), with more to see and do than in Donostia-San Sebastian. D-SS can be easily seen and enjoyed in two days, it´s a small city and not that many ammenities. Take into account that this is a rainy area any time of the year, and beach weather may not be an option in October.

This route seems to be coming up more.

Last month there was a similar question, Here is a link to that thread: https://community.ricksteves.com/travel-forum/spain/tips-d21599c3-4932-47aa-8084-2f0a60e71276

As Jules noted, how you travel is important. I suggest taking a train to Zaragoza. Spend a two nights there. If you have a car, you can go to Belchite. It was the scene of some fierce battles in the Civil War and they have left the town as is as a memorial.

How many days depends on what you want to see in each place. The sites in the places you seem to want to visit are not tourist traps. They are popular destinations for a reason. Determine what you want to see in each place along with how long you think it will take to see it. Total everything up and you’ll get an idea of the time you’ll need. Invariably you’ll spend more time at some places and less at others. Don’t visit a place just because somebody says “it’s must see.” Visit places of interest to you.

Belchite was mentioned as a side trip from Zaragoza. It is worth visiting but check out the times when tours are given. We missed the tour time, but fortunately we had a rental car and drove along the town’s fence line on a dirt path/road and we’re able to see everything; just not as close as we would have on the tour.

Other Spanish civil war sites near Zaragoza include, Monument to the fallen Posición de San Simón, Las Tres Huegas, Guerra Civil trenches, and Orwell Road (trenches, command center). They are all very near each other.

Outside of Madrid is yet another vestige of the civil war; the penal colony at Bustarviejo. It’s a civil war era penal colony for railroad workers a mile outside of town. It is best reached with a car.

Would it be advisable to add an extra leg after San Sebastian/Bilbao to Oveido, then to Madrid from there? I was thinking of exploring a town like Valladolid or Burgos after Basque Country en route to Madrid, but from what I've read, it doesn't seem like there's much to see/do there. I'm happy to be proven wrong though.

In all my trips to Spain, nothing about Valladolid drew me to it, so I’ve never gone. I’ve been by it on numerous trips. Burgos is very nice. Its cathedral, with the tomb of El Cid and his wife Jimena, is beautiful. Up the hill from the cathedral is the Mirador del Castillo that affords a great view of the city. Most things to see in the historic center are by the Plaza Mayor and Plaza Rey San Fernando. The Santa María arch is pretty and there’s a nice tree lined park that goes along the Río Arlanzón (more like a stream). A short walk away along the river is the monument to El Cid at Plaza Mio Cid. The Camino de Santiago goes right past the cathedral so you’ll probably see pilgrims in the plazas.

A few kilometers from town, but walkable is the Cartuja de Miraflores; a 14th century palace turned into a monastery. It’s the Royal Pantheon of the parents of Queen Isabella the Catholic, John II and Isabella of Portugal.

With a rental car there are a number of small villages worth visiting within an hour of Burgos.

Burgos makes a very good stop on the way from Bilbao to Madrid, its gothic cathedal is absolutely a must-see and its old quarter is very nice. The Museum of Human Evolution https://www.museoevolucionhumana.com/en/ (as nearby it´s Atapuerca, one of the most relevant prehistorical sites in the world, https://www.atapuerca.org/en/apartado/1989/sierra-de-atapuerca-sites

I totally agree with the others that Burgos is worth the visit.

Going to make a suggestion that instead of a leg after San Sebastian, a leg before. If you have a car leaving from Zaragoza, you could visit Huesca, Loarre castle and San Juan de la Peña, stopping in Jaca. Then cross the Pyrenees and go to Pau. Then head out to the coast, see Bayonne and San Juan De Luz before going to San Sebastian,

If you have a car, Asturias is fantastic to spend as long as possible.
Oviedo makes a good starting point.
I found that area more interesting than Burgos, as within easy reach of Oviedo you have mountains, coastal villages, some of the best restaurants in Spain.....

Valladolid (and environs) is an unsung gem...but probably off your true for this trip.

For food lovers, carnivores, that region would be in capital letters.

Congrats on your upcoming trip! I have traveled extensively in Spain and spent 2 summers studying Spanish in Salamanca. I know how difficult it is to decide where to go. I prefer Madrid over Barcelona but if Gaudi interests you then Barcelona is a must. My thoughts:

-Consider a day trip from Barcelona to Figueres where you can tour the Dali Museum. He designed it himself and it is unique and eclectic. Cadaques is a charming town on the water. Superb food and ambience.
-3 days is plenty in San Sebastián. One of my favorite cities and a definite must. Bilbao is a wonderful city. Check out the Guggenheim museum.
-Day trips from Madrid are a great idea. I suggest doing Segovia and Avila. Avila is a quick few hours, just see the city wall and the cathedral to view the St. Teresa relics. Segovia is great for ambience. Make reservations to enjoy the roast suckling pig. La Granga is also close. I wouldn't pay to go in the palace but the grounds are beautiful. It's like a mini Spanish Versailles.
-Think about Salamanca. The architecture is stunning. Spain's oldest university is there (founded 1118 I think) and the Plaza Mayor is beautiful. The food is great and reasonable because of the large student population.

-In Madrid you can't miss the Prado and the Reina Sofia. Picasso's painting "Guernica" is a masterpiece depicting the bombing during the Civil War.
-I highly recommend a day trip to El Escorial de la Lorenzo and the nearby Valley of the Fallen. The latter is the most haunting memorial I have ever seen. A must for those interested in the Civil War.

-Besides the day trips I would plan 2 full days in Madrid at a minimum. So much to see and do. Make sure to visit the bullfighting bar at Plaza Mayor and get churros and chocolate.

Have you ruled out Andalusia? Granada and Seville are great. It's hard to say how many days for each stop because it depends on your priorities. Considering your interest in architecture I would consider Cordoba. It is a fascinating building. It was a Visigoth temple ( you can view remains of the foundation), then a Muslim mosque, a Jewish temple and then a Catholic cathedral. Stunning building.

Things I learned from my last trip with my husband...

-Stay overnight at least in the city you fly into in Spain. I made the mistake of flying from Madrid to San Sebastián upon arrival and we were miserable in the airport waiting for our flight. Jet lagged, cranky and hungry.

-Utilize the train system. The high-speed AVE train is great. We did a few legs flying and they were fine.
-We didn't rent a car and had no regrets. Some of the smaller cities are a nightmare to drive in. In fact I think Toledo has banned car in the old part of the city.

Lastly, get your Spain guidebook by Rick Steves, divide it into the places you visit and use it as a great resource. He and his staff have never let me down. :)

Excellent ideas from skyklad. A couple comments. I think she mispoke, she means the Mezquita in Cordoba is wonderful, interesting, unique (just about every positive attribute!) Cordoba is a wonderful city to stroll as well. She recommends staying overnight in the arrival city and makes great points. It also allows you to buy the AVE train tickets in advance for a substantial discount. That said, it adds an additional hotel change. I also like the idea of a quick nap on a train so I can arrive a bit rested to do some sightseeing. Lastly, I always make sure to stay at least one night at the city, I will fly home from. You never know what can happen if you need to travel to the airport from a different city, the day you need to fly home.

But with only 17 days, I don't think Andalucia fits in to the plan...although its a quick train ride from Madrid to Cordoba....

I'd want a minimum of three nights in San Sebastián, but I've taken trips where I spend a week in that city and never get bored. I think a lot depends on your interest in food.

You already have Madrid..4 nights minimum, I think
Barcelona--another 4 nights minimum
Day trips from Madrid=another two days in that city

So you are up to 10 days with that alone.

Plus three nights Bilbao, leaves you with four more nights if my math is correct.

I would not go to Andalucia for only four nights, but others have offered some good suggestions: Burgos, Oviedo,
etc. Valencia might also work, but I would just add days to the basic plan...

I'd go to Oviedo only if you were wiliing to rent a car at the airport or train station (we did the latter, and returned it at the airport, which is pretty far from Oviedo city). and drive to some of the more rural towns and scenic places.

We spent about three weeks in Asturias alone, with an only small one-night detour to Astorga, in Leon province. But you'd need a car to see Asturias properly and in a short time.

Are you ok with flying into one city and home from another,, to cut down time retracing your steps?

I wanted to fly into Barcelona and out of Madrid, but the price difference was too great. I'm leaving out of Barcelona as well in this case. My departure flight isn't until 2 PM, so I think I can make it if I leave Madrid early