California: Universal Studios and Disneyland are loads of fun. The beaches in San Diego are amazing. Legoland is pretty cool if you have kids or still are one at heart.
Vegas: Lots to do. If you're over 21, naturally you'll want to gamble. I suggest staying on the Strip for that, and gambling in the big casinos. Not only are your odds better on things like the slots, but if you're into card games, it's a much more fun and social experience. During my birthday last week, I lost $100 during a game of Blackjack, and I hardly regret it because of how awesome the people I met were.
As far as clubs go, the ones on the Strip are pretty expensive, but they're usually pretty awesome. There are cheaper alternatives off the strip. In terms of bars, don't bother with them on the strip, it's just overpriced drinks. The bars in Henderson are cheaper, and often just as social at night.
You're going to want to see some shows. I recommend a comedian (there are always at least a dozen popular comics performing here any given night) and a concert if your favorite artist/band is in town.
Check out the buffets here! Nobody believes me until they try them. The buffets in Vegas are not like anywhere else I've had in the world. Atlantic City doesn't hold a candle to the food here. Buffets can be hit or miss though, so I recommend: Bellagio, Wynn, Wicked Spoon & Rio.
The food in general here is really, really good. Lots of fine dining choices.
Strip clubs are pretty big here, if that's your thing.
Lastly, don't be afraid to meet new people. I think the best part about nights on the Strip is just talking to random people and having a fun night together.
Of course, be sure to check out Spinetti's and Gambler's General while you're here for cards.
There is also downtown Vegas which is a sweet experience as well. It's kind of like the Strip, but with tons of other stuff to do.
