There's nothing more calming than sitting by a river and watching the water as it makes its way downstream. Why not pack a picnic lunch and spend a leisurely few hours at Estuary of Bilbao?
When you've finished up at this flowing body of water in Bilbao Bay, why not make your way to Sopelana Beach, Vizcaya Bridge, and Arrigunaga Beach? They're near Estuary of Bilbao and also popular with many travelers.