I've done San Sebastian heading North to Poitier not all that long ago.
The CAS / DAs on the face of it are very intimidating. In practice I found that they are not on all the time, so a "night before" check on NOTAMs created a reasonably straightforward route through - I pretty much just did 2 straight lines - defined by a dog-leg inland to avoid the military airspace around Cazaux. The local controllers were also very helpful.
So my advice would be - plan a reasonably straightforward route, check NOTAMs, talk to ATC, and things should go fine.
Incidentally - I've not been to Biarritz, but San Sebastian was a lovely little town, friendly and helpful airport - lots of nice places to stay, eat and drink there. It being Spain, don't forget you'll need your flight plan, licence and passport every time you want to return airside.
G