Day 1: Casco Viejo first thing, then river, Arenal and Ensanche — orientation before the wide grid.
Day 2: Guggenheim first with a proper slot, exterior then interior; estuary walk and slower afternoon in Abando/Indautxu.
Day 3: Artxanda by default when it is clear; bad weather → indoor day — then lived-in Bilbao and a practical finish, not a last-minute random excursion.