In the other thread you mention being in LA a couple nights then going to Vegas for your birthday and returning to La the 16/17. I hope you aren't planning on spending that whole time in Vegas, because after a few days that town gets *real boring*.
However, while in Vegas make sure you have breakfast at the Paris casino, they have a breakfast buffet that is fantastic. I kid you not, it's great. It's of course a French style buffet, but you travel between the various chef stations and order what you want and they make it for you right there. None of that "Vegas bacon" (get bacon at any buffet in Vegas and you'll know what I mean). Breakfast at the Paris casino would be a nice birthday kind of thing.
If you are into kitchy kinds of stuff, nothing beats the Liberace museum. It's off the strip, but worth the trip in a very wierd kind of way. They've got his pianos, costumes, cars, etc. It's a laugh.
Your birthday night in Vegas should probably be taking in the Cirque Du Soliel show. I think it is called "O".
hoover dam is about a 45 minute drive out of Vegas, and there is a national park west of town that is really quite beautiful. I think it is called red rock park or something like that.
Now on to LA. My taste will probably be considered quite boring, as I'm not much for hanging out in clubs and getting drunk. LA has a great museum of fine arts with a unique Japanese wing. That would go well with the sushi mentioned in the other thread. Also worth a look for a first time visitor is the LA tar pits right next to the LA Museum of Fine Arts. There's a small museum about the tar pits on the same site, kind of interesting and a good look at what was going on in LA during the last ice age.
I'd also suggest going to the Santa Monica pier. There's an amusement park, plus great walks on the beach and vistas of the coast looking north and south. Lots of nice restaurants and shopping on the 3rd street promenade (must think of the girfriend as well).
Of course, a visit to Sunset Boulevard is required for the first time in LA. Take a look at the Whisky and the Rainbow while you are there, or maybe even stop in for a drink. For your classic original 50's diner experience make sure you stop at Mel's on Sunset Boulevard.
Another thing that is fun to do is to drive around Beverly Hills and look at the homes. Kind of mind-boggling actually. The most expensive homes are north of Sunset Boulevard west of the strip. The "flats" where many old time movie stars lived are located between Sunset Boulevard and Santa Monica Boulevard, west of Doheny. While in Beverly Hills make sure to stop off at Rodeo Drive. A restuarant I like near there is called "Il Forniao".
As has already been suggested, a drive up the coast on PCH is very nice. Stop off your first night in San Luis Obispo, a quaint old fashioned California town. Then, a little ways up the road is the Hearst Castle, a *must* see if you take that drive north. There's a hotel/motel a few miles up the road called the "Ragged Point Inn" or something like that. They have a web site. Anyway, the hotel is on a cliff overlooking the ocean. Romantic and quiet with incredible views, the girlfriend will love it.
Okay, there's a few suggestions. Have fun!