A few recommended spots to enjoy delicious food ( pintxos ) in Parte Vieja of Donostia - San Sebastián:

Bar Sport - don't let the name deceive you. This is not a sports bar. Try the Foie Gras and the Txipiron relleno de Txangurro (grilled baby squid filled with crab).

Borda Berri - only serve kitchen-prepared pintxos. Try the Kebab (not really a kebab but, rather, a delicious pork rib) and the Risotto Idiazabal.

Casa Urola - with a slightly more upscale feel, try the Vieira Ajoblanco (scallop) and the Urola (a delicious lobster salad spoon).

Itxaropena - perhaps less well-known, but stellar. Their Volcan is a beautiful meaty dish with egg yolk and for a vegetarian option - try their artichoke and tomato salad.

La Cuchara de San Telmo - a narrow, often crowded, vivacious place hidden in the outer limits of Parte Vieja. Only serve kitchen-prepared pintxos. Recommend the Navajas (razor clams bathed in a delicious mint/lemon sauce) and the Cotxinillo (suckling pig).

...it is very rare to find poor quality food in town, so explore, branch out, discover, and whether you are a couple, single, swingers, poly, ENM, ONS, etc. - enjoy your stay and your play.