Just in case you plan to stay in Donostia-San Sebastian for the beach, take into account that it´s a rainy city and beach weather is not always (and less in May) a possibility. It´s a working city of 185,000h with three beaches, but do not expect any amusements, bars or anything near the beach as it´s very far from being a beach resort, water in May will be quite cold. A beautiful city with great food atmosphere, but just wanted to let you know, as many people think that D-SS is a beach resort town...and it´s not the case.
Also, bear in mind that lodging in D-SS is expensive any time of the year.