Bayonne, France for 4 days (also to San Sebastian, Spain which is basically right around the corner). What are the cant miss things to do and eat?

I plan on eating my weight in Paella as I love the stuff, but what else?

We plan on going to see the Gaudi sites, as well as the Mercado de La Boqueria.

Same question for the Basque country area? I am very excited as I've been all over Europe and France, but never Spain or that southern Basque area of France.