I agree wholeheartedly with Laurie. Santillana makes for fabulous pictures, but it basically is a tiny pedestrian street. It has an albergue in a brautiful super old house at the entrance of the pedestrian street Cason Solar de Hilagos, and a restaurant with delicious menus del dias, Pasage de los Nobles, But in an hour, even less, it will be a matter of "been there done that". Arrive early-midday, hike up to Altamira, wait for your tour to start, visit the cave replica and ealk back down for dinner. OR... if you arrive on Friday am run up to Altamira to run a chance at winning the draw to visit the real cave!
Santander is a city that took a major hit during the civil war, so a lot of its architecture is mid-century, but a visit to the Magdalena, the royal family's former summer residence, and set of the series Grand Hotel is a nice visit. The muni albergue near the cathedral is a tight fit, hope the person is the bunk next to you has showered upon arrival.