Get a dose of vitamin D at Sonabia Beach. Once you've topped up your tan, hang around and watch the sky change color as the sun begins to set.
You're spoiled for sightseeing choice when in Santander - Cantabrian Coast. Sonabia Beach is a great place to begin here, but there are plenty of other worthwhile sights to explore. Orinon Beach, Playa del Regatón, and Mount Buciero are near the stretch of coastline and will fill in your day nicely.