You are probably right about skipping Bilbao - apart from the
casco viejo and the
Catedral de Santiago the only places worth visiting are the
Plaza Nuevo for one of the finest self-contained
txiquiteo in the whole of northern Spain. Of course the
Museo Arqueologico, Ethnographico e Historico Vasco is interesting if you like that sort of thing and the
Museo de Bellas Artes with its collections of works by El Greco, Zubaran, Goya, Gaugin and Bacon may be worth an afternoon.
The Guggenheim is really all about the building so you can skip the collections apart from the Picasso's; Miro's & maybe the Kandinsky
.
Sorry
@Dchell , you only have 6 days don't you. It's probably obvious I'm a bit of a Bilbao fan but Santallina is definitely 'quaint'; Llanes is one of my favourite little port / resorts on that whole coast: and Boo / Bo / Po / Poo should not be missed if only for the photo-opportunity.
Enjoy what you can, when you can and never fear that you might have 'missed' something. You will see, hear, taste, smell and experience so much on that wonderful way that you will not have 'missed' any diversion. Though the
Cueva Tito Bustillo will be worth the diversion for some
Buen Camino!