In that case: I really enjoyed the free English-language tour offered at the Bilbao Guggenheim at about 12:30. You go to the info desk to get a little sticker which is your ticket for the tour. Capacity is limited, so you want to hit that desk well in advance. The info people warn that it's not a tour of the museum collection, more about the founding of the museum, but I found that not to be the case (perhaps varies by guide??). My tour did, indeed, discuss how the museum came to be in Bilbao and the various construction materials, but it also covered the lower floor pretty comprehensively. Afterwards, I walked back through those exhibits very briefly but otherwise concentrated on the top floor. I don't remember the running time of the tour but am pretty sure it was 60 to 90 minutes. With a serious interest in the museum, I'd be tempted to show up by 11 or 11:30, try to pry from the info desk folks an assurance that the tour does not visit the upper floor, then head up there. You'd be able to cover a good bit of that floor while waiting on the tour, assured that you had a spot on the tour.
Of course, also check on the status of the Bellas Artes Museum. The T.O. right beside the Guggenheim should have current info. There's an announcement on the museum's website, too, which one hopes will be removed promptly when the museum re-opens.
The Casco Viejo (old quarter) is picturesque and worth a wander. I also enjoyed walking along the river and through the "downtown" area with its many handsome 19th century buildings. Rick recommends the boat tour on the river, but I never found time for that. I opted to go to Bermeo, a picturesque town on the coast. Took a train, probably the FEVE, which is slow but goes through pretty countryside. The Basque Country is so, so much more attractive than the country around Madrid. Not speaking of the many great side-trip cities accessible from Madrid, of course.