Catalina Island. I haven't been, but I hear it's fabulous.
San Juan Capistrano has a nice mission to visit and lots of good places for lunch. And it's just a beautiful town.
I've always wanted to go to Venice Beach, but I just never seem to make it there when I'm in the area. The little shops, street performers, artists, roller bladers, etc., are supposed to make the place unique and a bit eccentric. There are a lot of TV and film scenes shot there.
If you go to San Diego, you also can go to Tijuana. There is not much to see there, but it's great if you like to shop. And there's the Agua Caliente Race Track and Casino if you like greyhound racing and/or gambling.
I'm trying to think of more.