But I'm biased, as I just did that for my honeymoon last july (sans Madrid).
San Sebastian is great for food - from the pinxtos at all the various local bars to the world class restaurants (arzak & mugaritz namely). Mugaritz remains my favorite dining experience of all time. Such a great meal/experience and worth the cost, in my opinion. Outside of the food, the beaches in San Sebastian are ok as welll.
Barcelona is really interesting. Sagrada di Familia is a must to visit - be sure you buy your tickets in advance so you don't have to wait in line. Obviously the gaudi architecture is really cool, and barcelona itself is pretty uniquely designed for a major city. Food wise, I cannot recommend Albert Adria's Tickets restaurant enough. Reasonably priced, unbelievable food. Fun atmosphere as well. I'm not sure if they changed their reservation policy, but it used to be their reservations go online on their site for 3 months out everyday at midnight barcelona time. A bit tedious, but well worth it in my opinion.
If you're looking for guided tours/locals to show you around - San Sebastian Food is a great company (sansebastianfood.com). We did their pinxtos bar crawl and it was a lot of fun. For Barcelona, someone on the back room (think it was Milhouse?) recommended Hi! This Is Barcelona and they were a great tour guide as well. (http://www.hithisisbarcelona.com/index.php)