San Sebastian, or Donostia in local Basque, is one of the most touristic cities in the Basque Country. It is known as the "Pearl of the Cantabric", for its bay and the beaches of La Concha (the shell), Ondarreta and Zurriola, for surfing. Also it has been worldly known for its gastronomy. You can find top quality chefs and more popular bars in the old town where you can try local pintxos (the basque version of tapas) so you can try great dishes in small bites.
Apart of its bay and gastronomy, Donostia has a very living culture, so you will have the chance to visit museums, the old town with the traditional harbour, unique architecture in downtown and sea walks such as the Paseo Nuevo or Peine de los Vientos (Comb of the wind).