Bilbao

It depends on your interests and pace of travel. The main draw, in addition to just the general city, is the Guggenheim, that will take half a day. There is a Maritime museum if that interests. Wandering the town can be done in an afternoon or a couple days, and the food scene and pintxos bars are popular, especially toward the weekends.

I would not stay less than two nights, three could be done easily, more than three, then you are probably looking at some things outside of Bilbao and nearby, or just into kicking back and relaxing.

We just spent 3 nights in Bilbao last month and were able to see everything on our list. I highly recommend a day/half-day trip to Gernika. It was an easy train ride.

As a Bilbao local...we have a beautiful, lively, big, old town. The Azkuna Zentroa. The funicular train. The new (19th century) area with its wonderful arquitecture. The bars, so lively, full of pintxos. The food...the wine...the lively streets...And then, you can go outside the city to the beach area (Getxo) and visit the unique Puente Bizkaia (1893 Hanging Bridge, UNESCO World Heritage). Three days at least.