I wouldn't go into Mexico if you only have one day. But if you have a car rental and a couple hours to burn, Lou's Records in Encinitas is worth the trip. And then a little further, and you can visit Legoland in Carlsbad, but if you only have one day, I think you're better off spending it at the zoo. The "...like all things in San Diego, its just nice to be outside..." comment is spot on. San Diego is my second favourite place on earth that I've ever been (Hawaii being the first). Perhaps browse local record stores Sunday a.m.?
I also second the Old Town suggestion. When I went there on vacation, we had dinner every night in the same place. Don't remember the place, though. It was 10 years ago anyway, so probably best to just find a 4.0+ place and eat there.