I'm jumping in late to this thread, so I don't know how much time you have and some other details. Here's my 2 cents. By way of perspective, I have lived my whole life just south of Los Angeles and have travelled to most of the places mentioned above.
If you've got the time, the drive from San Francisco to Southern California is not to be missed.
I would not skip San Diego if I were you. Coming from AU, I don't think any California beach is going to do much for you, but there's SO much to do in San Diego. The Zoo is excellent (there's also the "Safari Park" -- formerly the "Wild Animal Park, which is an hour north of SD but is much better IMO). As someone said, SF, LA, and SD are all very different places (frankly, I think LA is the least interesting but I've lived in the area my whole life).
I don't know that I'd go out of my way for the Grand Canyon if I were you. It's very cool, but it's very out of the way, if all you're going to do is stand at the edge and look. If you had a couple extra days to hike down into it, that would be different. An air tour out of Vegas will run you a few hundred $$ per adult. Bus tours will be much less but will take MUCH longer because it is so out of the way. I would recommend renting a car in Vegas and doing the drive yourself before doing a bus tour.
I would pick Memphis over New Orleans, but that's only because I've been to NO (although it was 22 years ago). If I were you, I'd probably go with NO, but I don't think you could go wrong either way.
I'll throw in a pitch for Boston too. Very different than New York, but equally as cool. Tons of stuff to do from American history focused things all the way to modern art and Berklee School of Music, the birthplace of Majesty/Dream Theater! And, if you're at all inclined to take in a baseball game, Fenway Park is the place to do it!
As others have said, I'm happy to chip in with suggestions. Enjoy!
<message edited by pg1067 on Wednesday, February 01, 2012 7:44 PM>