I've ridden all the way along the North coast. Campsites are plentiful but it depends on what time of year you're going as to whether they'll be open or not. There is the option of cheap rooms if you feel like it, more abundant in the larger towns. Roads are generally fine if you stick to the smaller ones as much as possible - where there's the option. There were occassions when it was easier/necessary to use main roads (not motorways), which were of course busier but as long as you're careful there's no problem. They're pretty friendly in that part of the world, fellow cyclists and drivers (in my experience).
Tour ideas - depends how long you're going for, budget, where you're getting the ferry back from etc.
A ride from Santander East through Bilbao and then up the French coast, getting back to the UK via a ferry from St Malo, would be a nice ride. Or, as Tim suggests, venturing into the PIcos would be superb, I expect. Depends what you#re after really.