This is my favourite part of the world so: San Sebastian is nice but we have only gone there on day trips which has always seemed plenty. Super for Pintxos (like Tapas) and if you find a good place which explains how to go about it you could have many a good few hours discovering all sorts of pintxos and the lovely icy white wine which goes with. The best ice creams are to be had at Loco Polo, where you choose your ice cream on a stick and have it fully or half dipped in any chocolate and any sprinkles! Remember that sometimes your eyes are bigger than your stomach.
Santander if you have the chance to visit is a fabulous small city. Really friendly and relaxed with the best beaches at Sardinero and definitely competes with Biarritz for my favourite place to be. It's about 3 lovely miles between the seaside (Sardinero) and the city centre but it is along a busy port channel so a lovely walk. The Spanish, in my experience, are so chilled and friendly. Nothing is a problem (I don't speak Spanish unfortunately)
St Jean de Luz is pretty enough but very geared towards tourists and very busy.
Biarritz is where I want to live forever. It is a lovely, relaxed (it is French though so relaxed re French and relaxed re Spanish are two different animals, Despite the proximity) town. Really great atmosphere and friendly welcoming people. We stayed in Bidart (South of the town) which is a 5km walk to the town centre.
Inland of Biarritz is interesting. Some of it wealthy and touristy but St Jean pied de Port was an eye opener: a pretty town but definitely much less affluent than is coastal neighbours.
Hossegar and Cap Breton are popular but I really did not like them. Very commercial and you could be anywhere in the world.
Arcachon is worth a visit for the sand dune (wear good shoes and bring lots of water, the sand can be boiling) but the town is not all that. However friends spent a few weeks in a campsite nearby and did enjoy the area.
Lacanau Ocean I did really like. It's new and atmosphere-less (for France which is swamped with stunning villages which barely feature on a map) but it has a fantastic beach (for miles) and it is (unusually in my experience for this particular corner of France) good for family cycling. There are woodland trails to the inland lake and the roads are safe. It wouldn't be for every one but it is a decent family holiday destination I think.
North of Lacanau is stunning. I'm talking About towards Soulac-sur-Mer. Between Biarritz and Arcachon and then on to Lacanau is a hard drive. Two lane road through miles upon miles of flat grassland. It's understandable how this area has only been developed in the last century. And that surfing contributed to it's success. It's flat with a straight coastline, so great surf but lacking in any interest. Surprisingly from Lacanau on there are lovely tiny villages which are inhabited and cared for but not touristy. We took the ferry from soulac to Royan and were really impressed by royan. Wish we had spent more time there.
West of Royan is a fairly wild coast but lovely in its own way.
Now I know you said Bordeaux so I went a bit out of your way. Coastally I would bother with more beyond Biarritz. I have it even been into Bayonne but it looks lovely (also planning my next trip!)