So, a little trip report that will hopefully be helpful. I like to open with a bit about who we are and how we travel because I think that helps people evaluate what is worthwhile to them.
We're in our late 30s/early 40s, active, living in Europe for 7 years and my husband has decent Spanish and I understand some but have lost the ability to speak with any decency (but do OK in restaurants). I'd been to Barcelona and Madrid before, and Mallorca a handful of times, my husband had only been to Mallorca in May.
Our time in Europe is limited, so my husband requested an active trip focused on seeing as much as possible, and wanted to rent a car to see things not possible by train. I was skeptical but this was "his trip" in many ways and so I checked in with him while planning and he OKed the pace. We survived, but we both agree we would have cut out at least one of the one-night stays. We are active, accustomed to walking, and we found the pace of this trip a bit punishing at times - keep that in mind when planning your own trips. In my experience you will almost never say, "We spent too long in Place X" but rather "I wish we'd slowed down for another night in Place X."
We traveled open jaw, flying into Bilbao from Stuttgart via EuroWings on Sept 2nd with only carry-on luggage (I'll talk more about that later). We picked up our rental car from the airport, drove to San Sebastian, parked in a free lot outside of the center of town for 3 nights (also more about that later), then got the car, drove to Bilbao (1 night), Polientes (1 night), Llanes (2 nights), then to Leon where we dropped off the car to take the train to Madrid for 3 nights before flying back to Stuttgart.
When planning the trip I'd really hoped to do it in reverse - fly into Madrid for a few nights, take the train up to the north, pick up a car, and make our way towards Bilbao, relaxing for a few days in San Sebastian before flying home. Unfortunately this just didn't work out with our timing/budget. We're not strict budget travelers but we're definitely in need of good prices and I couldn't find them with the alternative, unfortunately.
We stayed in a wide variety of lodgings, ranging from a 36 euro/night pilgrims' hostel (with en-suite bathroom) in Leon to a quite posh cinema-themed pension in old-town Bilbao for 95/night. Unless I'm doing a beach trip I'm loathe to spend more than 80/night for a room but I do like comfort and class when I can find it. Despite the range in prices I was pleased all the places we stayed at on this trip and would recommend them all.
I would probably not recommend renting a Fiat 500 as a rental car. My husband thought it would be "fun." I told him we were going into the mountains. Mountain roads. Inclines, curves, etc. He still wanted the Fiat. I said fine. He regrets the Fiat. The size wasn't a problem (we flew budget, after all!) but rather the power of the car.
OK, on to the trip itself!