several: Casas Rurales or rural houses for rent
they have an assoc and official site, you can browse in Spanish, I have used them in the past and a great experience.
http://www.casas-rurales.info/paises/espana/cantabria/
Potes is nice but I stay in Santillana del Mar as had friends to visit there. Picos is a wide area, we just drove by it to Gijon to again see friends. you can see casas rurales or rural house there on the link above too, see Asturias.
However, at the picos de europa link there are casas rurales listed too
http://www.picoseuropa.net/turis1/turis1.php
check the destination link above right in VT for tips by posters like us. Hope it helps your planning.
Cheers
real living by the locals in small quaint towns, several choices, Santillana del Mar is gorgeous.



