Flying into Barcelona, home from Bilbao...where to go in between?

San Sebastian is definitely "worth the trek!"

If you're enthusiastic about food, San Sebastian is more than worth the trek. It is unparalleled.

I take it that you are an experienced European traveler and know you are not significantly slowed by jetlag and sleep deprivation after overningt flights. Otherwise, you are giving yourself just one non-jetlagged day in Barcelona, a city which is difficult to blitz because it has so many sights essentially requiring pre-purchase of timed tickets. I believe you'll have difficulty with the short time you're planning there even if you are a good overnight flyer.

The train from Barcelona to Seville takes 5-1/2 hours, and by the time you traveled to and from the airports and showed up at BCN 2 hours early for security procedures, I don't think flying would be faster. So one night in Seville would give you virtually no time at all in what is a large city with quite a lot to see. I think you need a minimum of two nights there, and I wouldn't spend that much time in transit to give Seville such a quick look. On the other hand, the October weather will probably be quite a lot nicer in Seville than in tne Basque Country.

The bus from Madrid to Bilbao will take over 4 hours (could be closer to 5), and that doesn't include the time you'll spend getting to the appropriate bus station.

There are frequent buses from Bilbao to San Sebastian and I believe also between the Bilbao airport and San Sebastian.

I don't kmow exactly how many nights you'll have on the ground in Spain, but even if it is 10, I do think you've got too many different, and widely scattered, destinations--4 base cities and at least 3 side-trips.

Way, Way too much travel. Stop and savor the places that you visit.

You need at least 3 days in Barcelona and 3 in Seville as well as 3-4 in Madrid (day trip to Segovia).

I suggest that you reduce the number of places that you visit, for example:
Barcelona --Madrid-Bilbao

I agree with geovagriffith. Better choose 2-3 cities and spend more time to explore and really enjoy the destination. You don't have to do everything and visit every city. Leaving out some cities gives you a reason to come back.

We spent five days in San Sebastián, and then took a high speed train for two and a half days in Barcelona. We had been in France before San Sebastián. We loved San Sebastián for the food, the beauty, and the walkability. It was our first city in our first trip to Spain. Since we were flying home from Barcelona, and only had 2.5 days due to the train travel between cities, we prioritized our sights there.

My recs: Barcelona: Get an early ticket to Sagrada Familia, ASAP, plan your other special stops and plan for lots of strolling, with some fruit juices from La Boqueria, and read recs from acraven and Enric.😀 They were so helpful to me as I planned. We saw a lot in our limited time...but there is so much more to see, including day trips out of the city.

Take the high speed train to San Sebastián, booking early for best price. We stayed in the Gros neighborhood, which seemed quieter, but very close to the old town, and the beach was lovely, with occasional surfers. We saw beautiful sunsets every night. We also took the funicular to the top of Monte Igueldo, with spectacular views and the unexpected fun of riding on an old style amusement park ride of a wooden boat on the Rio Mysterioso! One part of that ride was reminiscent of Disney’s Pirates ride. We loved the pinxtos and walking all over San Sebastián. We also went up into the mountains for a cider house tour, which was an unexpected delight. Our family loved San Sebastián.

Then you could make your way to Bilbao, either directly, or stopping on the way and exploring. I think I remember that buses were the best way to do that part, rather than a train.

Whatever you decide, I am sure it will be a great trip.
Enjoy!
Laurie