Basque cuisine is very diverse and made with excellent fresh products.
Its base are fish and vegetables these are often cooked like a stew.
You will find some of the most famous recipes,
Marmitako (stew with vegetables and fish), Bacalao al PIL-PIL (cod cooked with olive oil in a crockpot), Merluza a la vasca or salsa verde (hake cooked with clams in a green sauce with parsley)
You can taste txistorra, it is a type of sausage with pork meat
Also you will see in many restaurants the famous Basque PINTXOS which its a slice of bread where is located a small portion of food.
To accompany these foods you can drink Txakoli (type of basque white wine)
So... Have a nice trip and enjoy your meal!