Biarritz recommendations

I was in San Sebastian three days ago. Definitely worth a visit, walk up http://www.donostiasansebastian.com/urgull.html there is a great museum at the top. Lots of good restaurants in the Ciudad Vieja. Bilbao is also great. Driving around is a nightmare so park on the outskirts. The local transport system is cheap, frequent and clean. Don't forget to walk along the river. If you want a good view walk across the http://en.wikipedia.org/wiki/Zubizuri bridge and take the funicular (every 15 mins) up the hill where there are several eateries. ( You can get a wristband to renter the Guggenheim again the same day)

You absolutely need to visit the http://www.grottes-isturitz.com/grottes-isturitz/visite_gb.htm, a Paleolithic painted cave about 30-40 minutes inland from Biarritz. You can grab lunch at the little Basque Café in the adjoining town, St. Martin d'Arberoue. I have no idea about the name of the café, but it's a town of ~300 people, so there aren't many.

Re the restaurants, if you don't mind the drive, try http://www.mugaritz.com/menu_en.html(chef: Aduriz) in Basque Country, Spain. It's about 1h - 1.5h drive, if I recall. Mugaritz is considered one of the top ten restaurants in the world, but you can book lunches at reasonably short notice. There's also http://www.aubergebasque.com/near Biarritz, but it's a bit too Parisian to my taste.

On rereading your OP: if you're interested in good atmosphere, San Sebastian (Donostia) is a lot closer to Biarritz than Bilbao, and the food and atmosphere in the pintxos bars (Basque tapas) is outstanding. Gastronomically, Basque country is one of the top regions in the world: their love of fresh produce, fish, and locally grown organic meat borders on obsession.

2nding NekulturnY : try the old San Sebastian, you'll enjoy great food near great spots (the beach, the port...).

I'd sleep in Biarritz and spend every day in San Sebastian, honestly. I've lived in Spain and travelled all over. This is one of the best small citites in Europe, food is incredible and far, far better than the tourist fare in "Boarritz"

Nthing San Sebastian! Also, last summer we rented scooters in Biarritz and drove along the coast for a few hours. We stopped for lunch in a little tiny basque town. It was a blast!