Teatro Arriaga is Bilbao's grand old theatre, standing at the edge of the Casco Viejo with an ornate facade inspired by Parisian opera-house architecture. The present building opened in 1919 after the earlier theatre on the site burned down.
Even if you are not seeing a performance, it is worth a look while walking between the old town and the riverfront. The square in front is one of Bilbao's classic meeting points.