My brother lives in San Diego and pre-kids, we used to go down and visit frequently. Now, we still get down there - just not as often as we used to.
I've been to the SD Zoo a number of times. I have to admit - it's a pretty awesome zoo. I've not been to Legoland, but I know it's in Carlsbad and there's an outlet shopping mall near there!
There's also the San Diego Wild Animal Animal Park (I think they call it the Zoo Safari Park or something now). Personally, I think the zoo is better but this is still pretty neat.
Another great place to visit in San Diego is Balboa Park. It's right near the zoo (depending on which entrance to the zoo you choose, you might even drive through part of the park to get there). There's botanical gardens, kids museums, the architecture is pretty fantastic, plus lots more. It's a great place to go to just kick back and relax.
Old Town San Diego is another place that's great to just wander around. There's some pretty fantastic restaurants there as well.
Coronado Island is beautiful but definitely not a full days worth. Belmont Park is a beach front amusement park, so kind of cool that it's on the water but might seem kind of chumpy after
Disneyland .