I love Santander, and the Sardinero beach is a definite highlight. It's on the Sea of Cantabria - the water is refreshing (read cold, unless you're Canadian, in which case it's normal...). It's organized, the walkers walk along invisible lanes, and everyone playing paddle ball stays in one area. It's clean, the sand is nice, and there are good public facilities - why don't all beaches have showers so you can rinse the sand off before going home?
Speaking of showers, it's rain-rain-rainy in Madrid today. Oh, how I would love to be on a warm, sunny beach, parked on the sand with a stack of magazines and a giant iced tea at my side.