Esperanza:
Last night I thought of something: If you and the girls are early risers you can visit the Mercado in the morning before The Guggenheim opens. A lot of bus lines originate/terminate in front of the Mercado and any of the side streets will take you into the Casco Viejo. The sights, the sounds and the smells in the Mercado make it worth a stop; and you will understand why I made a vow that if I ever return to Bilbao I will rent a place with a kitchenette (or cooking privileges). :p

On the Plaza Mayor in the Casco Viejo there's a Brass & Glass restaurant the name of which escapes me. You might want to stop there for Café o Chocolate and they have some fantastic pastries.