we did a bike tour through the area a few years ago. santillana del mar is definitely worth a visit, some nice beaches near san vincente good for surfing too
if you do have a bike or want to head out on some mountain walks get the cable car up the mountain from fuente de and explore the higher paths/routes up there (i rode up too on a separate day, but with all our touring gear the lift is loads easier!) some really good walking/mtb’ing and you can cross the picos on the jeep track and drop down to sotres/la hermida if you wanted to push that far. we climbed one of the nearest peaks to the fuente de cable car station – not too difficult a walk with a bit of scrambling near the top, worth it for the view of the naranjo de bulnes and the high plateau area
the cider bars are great and the queso the cabrales makes stilton taste like dairylea (goes great with the local red wine though!) – if you order a cheese sandwich expect it to be very ripe :lol: