I'm guessing you're taking a boat to Santander. I did the same last year but instead of heading west into the Picos de Europa I headed east into the Pyrenees. I made St Jean Pied du Port in France (western Pyrenees) my base and did ride outs from there. I chose a French base as the camping is a lot better, pretty much every town has a Camping Municipal which are cheap and well situated. As a treat I stayed a couple of nights in a B&B called Maison Dona Maria. It was 55 Euros a night for a quality room and a safe place to park the bike.
The riding was brilliant with the highlight for me not only the Routes des Grand Cols (taking in the Tourmalet) but the road both sides of the Col de la Pierre St Martin - it was like a race track. Fantastic. On the Spanish side the riding was also fantastic with the famous N260 easily accessible - it runs all the way from Sabinanigo to the coast (if you feel the need) and from the bit I rode it was a great road. If you do get to the coast I've read that the coast road through Tossa de Mar is brilliant with curve after curve and ending up in Barcelona. Be careful though if you do head for the Pyrenees. Don't underestimate the distance in getting all the way across.
I was into dirt riding some moons ago and have my eye on a dirt holiday in Spain in the future. I've read some Austin Vince (Mondo Enduro) stuff and he tends to base a lot of his stuff he does around a Spanish town called Tremp (or that area) which is on the southern side of the mountains - I don't think it's legal to ride off road in the Pyrenees proper (might be wrong about this).
I've never ridden there but heading west into the Picos should be great. I had a cycling holiday there about 7 years ago and it's a beautiful region. I was based in Potes which is a busy little small town. If you spend any time in this region make sure you try their cider. Fantastic stuff. Also, if you have time be sure to spend at least half a day in San Sebastian. What a beautiful city.
Finally, if you do head for either of these regions I can recommend the Michelin Zoom series of 1:150K maps (144,145,146, 148 for the Pyrenees, don't know the number for the Picos).
Check out my brief ride report for a better overview - hope you like it. Have a great ride