Bilbao spent most of a century as a city of blast furnaces and shipyards, on a river nobody wanted to look at. In 1991 the Basque Government asked the Guggenheim Foundation to build a museum on a derelict stretch of that riverbank and agreed to pay for it. The Guggenheim Museum Bilbao opened in October 1997, and planners have talked about the Bilbao effect ever since. The city is older and tougher than its museum: it was founded in the late 13th century, grew rich exporting wool and iron, and still has a medieval core of seven streets.
It's a compact city of about 350,000 people, which is why three days is the right length. Two days cover the river, the museums and the old town without rushing, and the third goes down the estuary to a UNESCO-listed bridge and the seaside streets of Getxo. The Bay of Biscay brings mild air, low sunshine and a lot of rain, so pack a waterproof and plan your meals around the bars. Bilbao eats pintxos, not tapas, and takes them seriously.
Day one is the old town, the market, the basilica on the hill and the funicular at sunset, day two is the Guggenheim and the modern city, and day three is the Vizcaya Bridge and Getxo. Start on a Friday if you can: the Guggenheim is closed on Mondays and the Ribera Market on Sundays, and a Friday start puts the coast day on a Sunday and keeps Monday free. If you land on a Monday, swap days two and three. The rest of the Basque coast is under More time, and Spain has plenty more behind it.