I was there a long time ago. As I remember the cathedral is quite impressive, and there is also a Gaudi building there. I think they have some special kind of cocido up there, too, which I would definitely try (a bit heavy on a hot summer day, but what can you do?)
Personally I would rent a car and explore the area. I heard that Astorga and Villafranco del Bierzo are very interesting, and probably a bit cooler because in the mountains, but I haven't had a chance to visit yet. You could also walk a section of the Camino de Santiago in there somewhere.