What do you like to do, generally? We go to San Diego every summer on vacation and act like tourists (my fiance's parents are from there, but we cant help it). I have to admit I went to Comic-Con last year, stayed about 4 hrs since her brother was really into it.
Anyhow, if it's open, take a tour of Petco Park. Great stadium to see. Eat at JSix in the gaslamp quarter if you're into food. San Diego's not a great food city compared to SF or Napa out there, but this place was the highlight of my trip two years ago (things can change, but I checked and it's still the same chef). I think the cheesy harbor tour on the boat is a lot of fun. The bus tour of the city is cool, especially for scouting to see where you might want to hang out later on. There are a few local breweries to tour, although I don't remember their names, but I had most of the local brews and liked them. Anthony's Grotto is a good but not spectacular place to get seafood, although most of it is fried and the interior has a diner feel to it. I thought driving up the pacific coast highway was fun, just to see the towns. Knott's Berry Farm is nearby if you like amusement parks (Disneyland is like 90 mins north too), although they got old fast for me, since I'm used to Great Adventure and Disney World. I can't remember much else right now, but just remember that San Diego's a laid back place and if you walk around the city, you'll find things to do.
Hey, hit the beach, too. San Diego's got 70 degree weather all year round. We sure don't have that in the northeast!