Nothing works for me! I have given up with the train idea because to get anywhere and back in a day requires an early start. ✈️ Flying Santander to Madrid is a bit more civilized time-wise (although I never got as far as checking how early you have to check in) but st around 566 Euros for a there and back in a day, I bailed. Decisions, Decisions. I think that is more than I want to spend. I've got some cheaper ideas. Only ideas, but I'm relaxing!!!
Car rental, is that an option? Madrid is 280-300 miles one way from Santander so probably not a practical day return. Similarly, by train, it's a 5 hour journey so again something one would consider for 2-3 days maybe. A bit closer but considerably smaller is Gijón, along the coast. A nice port town, check it out. Might be worth considering. A little over 3 hours by bus or less than two hours by car.