Your topic 'threw me' a bit. When I think of northern Spain I think of the north coast: Bilbao (where I would recommend Hotel Conde Duque), Santander, Gijon and a'Corunha. If you want 'active' visit Potes, about 25mi south of Unquera (Cantabria) in Picos de Europa.
But since you are already committed to San Sebastian, Mallorca and Barcelona, you may have to leave 'northern Spain' for your next trip.