There are a few older posts on here with info, but here are a few recommendations to get you started. My family spent time in Cantabria caving a few years back but did get up to Astigarraga for a few days. Astigarraga is a little south of San Sebastion/Donostia and has a bunch of different Sidrerias around the town. The times for meals and tours vary a lot, and we're a little difficult to navigate but I highly recommend visiting the
Sagardoaren Luraldea museum in town for a quick tour and to pickup some sidra glasses and related stuff in their shop. They can also make tour and meal
reservations for you through their website, but you can make them directly as well. Due to our schedule we were only able to visit
Alorrenea which is just outside town and pretty traditional, and
Iretza in town which is more modern. The town is very industrial so don't be surprised to find a traditional Sidreria next to an autobody shop. There are a number of good places in town to stay, but we stayed at
Pension Astigarraga which is within walking distance of everything in town and had an interesting display of sidra bottles inside.
In Cantabria we didn't visit any Sidrarias but did drink a bunch the various supermarkets had on the shelf. We also didnt make it out to the Picos ans Asturias, but they have a nice looking
museum and Sidrerias in and around Nava too. Claude's Cider Planet book has a bunch of good info too. Not sure where all your going, but here are a few intersting places we visited that are mostly on the way between San Sebastion and the Picos. Most folks at the hotels/museums/sidrerias can chat with you in English, but if you know some Spanish (or less likely Basque) it will go a long way to make your way around.
Feel free to reach out if you want any other non-cider related info on the area.