Bilbao from Madrid - worth flying in for a day?

The hottest travel experience for young people these days is the 24 hour mini-vacation.
Anything can be accomplished if you have the money, the ability, and the desire.
However, only YOU can decide if the experience is worth it.
Would I fly all the way to Egypt for just one day to see the pyramids? Absolutely not--I would need at least a week to take it all the marvelous sights.
Would I go up to New York City for the day to do some sightseeing and shopping? Yes, of course, I do it all the time!
My only caveat is that it is much easier to do this if you already know your way around, or if you highly organize everything in advance.
Just make sure you check your flight schedule options and airport security times, as well as opening and closing hours for the places you most want to visit. Travel back and forth to the airport should not be a challenge.

https://www.bilbaoturismo.net/BilbaoTurismo/en/bilbao-en-1--2-y-3-dias/bilbao-in-one-day

¡Buen viaje!

Although I have not been to Spain in some time, I consider Santiago de Compostella a much richer visit.

As a Bilbao local (and thanks so much for its correct spelling, instead of Bilboa or Balboa): the airport is small and just 15min from the city center, so access to the Guggenheim is almost inmediate after leaving the airport. I would dedicate one hour and a half to the Guggenheim Museum, then I would walk (Bilbao is the 4th most walkable city in Europe) towards the old town along the river or on the streets parallel to the river (architecture is beautiful). A first stop at Café Iruña is compulsory, then on to the Old Quarter (Casco Viejo) to enjoy the lively pintxos atmosphere around 1pm for a couple of hours before lunch. I would not miss the Ribera food market (8am-2pm Mo-Sat); the streets of the old quarter and the Santiago Cathedral square; the funicular train of Artxanda, for the best views of the city; Azkuna Zentroa, a former food and wine warehouse transformed into an amazing public civic center, with its very distinctive 47 columns showing a time in the history of architecture; and the area around the statue of John Adams, who was in Bilbao with John Quincy and Richard Adams back in 1780, during 9 days, architectonically very interesting. You may know that all taxes remain in the Basque Country and we do not pay them to Spain...this is one of the reasons Mr Adams came here to visit, apart from our constitution from 1452 (the old) and 1526 (the new). If you have time, our Fine Arts Museum (free entrance) is second to El Prado in quality of its exhibitions. The good thing is you can go walking anywhere.