My list of top things to do in San Sebastian include:
Walk the Promenade at La Concha Beach: It is a great walk along the ocean where you can see the apartments and hotels along the way. There are several cafes and restaurants overlooking the water and beach half way down. At the very end there is an Eduardo Chillida sculpture known as El Peine del Viento or The Comb of the Wind.
Sit in a café and people watch: There are some great cafes along Avenida del Boulevard bordering the Parte Vieja or old town.
Explore the Parte Vieja: Do what they call a Txiketeo, or pintxo bar hop, where you can sample the food at several places.
Visit the Museo San Telmo at Plaza Zuloaga 1: Located in a refurbished Dominican convent, this is a museum featuring historical artifacts and exhibits on Basque culture as well as a small permanent collection of art by Basque artists. I liked this part of the museum the best. There is a special exhibition space, where I saw a show dedicated to Fellini and his films. Check out the website for the current schedule. The newer wing, with its interesting contemporary gray façade, has a nice restaurant and café facing the square. The church of the old convent has some great paintings so make sure you visit before seeing the permanent collections.
If you are into fashion and want to do a side trip, check out the recently opened Cristobal Balenciaga Museum in Getaria, a short distance west of San Sebastian.
Stop in to visit the Cathedral Buen Pastor in Centro at Urbieto and San Martin.