There's really two main areas to hit up in Cali... northern california and southern california. Here's my suggestion:
Fly into San Francisco first. Enjoy all the famous sights including biking across the Golden Gate Bridge, taking a ferry off to Alcatraz, taking in the scenes of Chinatown, window shopping at Haight-Ashberry or castro. If you're a wine coinosseur, take a trip out to Napa Valley and go wine tasting, and dine at some of the best restraunts in the country. If you're more of the partying type, check out SFs red light district. All that I have listed is just a few of the options to do up north. But I would definitely say that this would probably be the "relaxing" part of your trip.
From here LA is a 6 hour drive (depending how fast you drive) down south. My point of view is that LA has less in terms of sightseeing, but for someone in their twenties, you'll probalby get enough of that in SF. In my experience, LA is known for their woman, parties, nice cars, beaches, and shopping. I think //M_owns_you has it down with his guide.
Finally Vegas is just a 3 hour drive from LA, and that's where you'll end your trip and fly back. Not much to be said about vegas, as I'm sure you already know what's there.
You could do the trip either starting at vegas and ending in sf, or the way I recommended it. It's just a matter of whether or not you like to end the trip with a hang-over, or in a more relaxed fashion. Hope this helps!
