The Real Sociedad football club is the pride and joy of almost everyone from San Sebastián. The most authentic experience you can have, at least in the sporting arena, in San Sebastián is seeing the txuri-urdin (white+blue) play on the home field.
If you can, catch a game in the Anoeta Stadium. Even better if it is the Bilbao- San Sebastián derby. Bring a bocadillo and get to cheering!