It is worth going on a tour with English-speaking guide to the Geggenheim Museum famous for its collection of modern art for at least 3 reasons (by the way, if you buy e-tickets via Excurzilla, you will jump the queue). First, you will see the museum building, resembling something between a spaceship and a rose and making stunning impression. Second, there are many masterpieces in the exhibition, including works by A.Warhol and R.Serra. And the third reason is the giant sculptures in front of the museum that deserve to be seen.
See also other sights in Bilbao: Park Güell, Aquarium Barcelona, Sagrada Familia, Casa Mila, Picasso Museum, Flamenco Show in Barcelona, Casa Batllo, Montserrat, Dalì Museum, Tibidabo