Help planning short trip to Spain--what to add to Pamplona?

If you decide to visit San Sebastián, definitely check out Bilbao for a few nights too, its the more grounded brother to San Sebastián. With its contrasting old and new architecture and nice riverside area, I actually like Bilbao better than San Sebastián. The main draw is the Guggenheim, one of the leading contemporary art museums in the world. I think your kids may like the whimsical and interactive art of the Guggenheim.

Alternatively, from Pamplona, one can travel south east to the medieval town of Olite. The Royal Palace of Olite was the seat of the Kings of Navarre. For me, it's one of the more impressive medieval palaces in Europe. There's even a Parador Hotel inside the Palace that one can spend a night or two, I think it'll be a hit with the kids! :)

If you continue south east you will reach Zaragoza, the capital of the neighboring region of Aragon. The city has more than 2,000 years of history including Roman ruins of the city walls, forum, and amphitheater, as well as the Aljafería; an 11th-century Moorish palace. The best part, Zaragoza is relatively undiscovered as far as major Spanish cities go. I would look into canoeing down the Ebro river as a fun experience for the kids.

From Zaragoza, one can take the AVE high speed train east or southwest and be in either Barcelona or Madrid in less than 2 hours for a flight home.

"I actually like Bilbao better than San Sebastián"

Me too!

"The main draw is the Guggenheim"

Well, it's certainly the most famous thing in Bilbao, and worth seeing if you like modern art and unusual buildings. But don't let its outsize fame fool you into thinking that Bilbao is a one-trick city. I like several of the other museums (the Bellas Artes and the Museum of Artistic Reproductions), as well as the Old Town (larger, more interesting, and less touristy than the one in San Sebastián) and the "downtown" (the area roughly between the Old Town and the Guggenheim).

Your kids (as well as you) may like the Vizcaya Bridge, a transporter bridge near Bilbao: https://en.wikipedia.org/wiki/Vizcaya_Bridge#/media/File:Zubia_jun.jpg. This clever animation shows how it works: https://en.wikipedia.org/wiki/Transporter_bridge#/media/File:MovableBridge_transport.gif

You can get there by metro from central Bilbao. There are only a few transporter bridges left, and this one was actually the very first built.