Wendy,

I went by bus to Santillana but I wouldn't recommend it in your case because there weren't too many buses a day. I imagine since it is pretty close( a little over half hour) that you could go by cab . If you do I would make the driver wait in Santillana as I don't know how easy it would be to find a cab there to bring you back.

I'm sorry I don't have any names of a private driver / guide that I can recommend from personal experience as I traveled by bus/ plane( I'm too chicken to rent a car and drive there!). I did find in the internet these 2 companies that provide car and driver service in the Santander area: HERE and HERE