The most stunning beach I have seen (although admitedly I have never been to the Caribbean ;), set…read morein El Sardinero, an iconic area in Santander, where you can also find a very distinctive building, the Casino.
This beach is something worth seeing, especially when the tide is down, and the First and Second Sardinero beaches are joined through the corner under the Piquio Gardens, and then the First Sardinero beach is joined at the other end with the Concha beach (Shell beach). You can then walk them all up and down, having 1km and 500m more or less to walk each way.
You can also walk the seaside walk all along it (and further more all the way to the city centre) without interruptions!
If you like a good beach, I thoroughly recommend this one.