It´s less than one hour from Santander. It´s an incredible prehistorical masterpiece and now you can enjoy the modern museum with a fantastic cave copy. You should take the bus to Santillana del Mar and from there 45 minutes´ walk. Anyway there are some buses to Santillana del Mar that take you up to the museum, check timetables. Another option is to go to Torrelavega and form there take a taxi. The museum is in the middle of a fantastic park and there are regular workshops (better check in their web site). In high season maybe you should book the tickets in their web site, better look for tickets with a guided tour.