San Sebastián has a tiny and pleasantly dated airport (EAS) located in the nearby village of Hondarribia. From there, you can take a city bus or a taxi, which will run you about 30 euros. Flying into Bilbao (BIO) is another easy option. A bus leaves from the airport exit to the center of San Sebastián every 45 minutes past the hour and costs about 16 euros.
Once you arrive, San Sebastián is best experienced on foot. The city can be walked end to end in about an hour (this is a favorite local pastime). City buses are plentiful, punctual, and straightforward. Taxis (+34 943 46 46 46) begin at 6 euros for a journey within city limits. San Sebastián is well-connected to other cities by bus and train, though buses are usually more efficient (if you have the option). The Renfe train connects to the rest of Spain, while the Euskotren connects San Sebastián to France and Bilbao.