I'm a country bumpkin. I'd take pride in my heritage as such if there actually was any actual heritage to take pride in. :) I'd like to visit Southern California for a few days with the express purpose of experiencing city life. Which city has the most active night life and is culturally rich? Like raves and clubs in decent parts of town, museums of art (I want to see something awe inspiring like "Early Morning, Summertime" or William Wendt "In the Valley" or something else worth hundreds of thousands or millions of dollars), concerts of major musicians, and like anything else that city life of twenty and thirty somethings normally entails. Only thing is I uh can't drink I am taking medications and stuff but I can be around alcohol without any problems i know just to carry a water or coke bottle with a lid.
Los Angeles, being a major city, has many cultural highlights (as you define them).
You will find a long list of museums on Wikipedia. I can particularly think of a few. The Getty Center, with a particularly interesting architecture and filled with art. The Museum of Contemporary Art, in downtown LA. And the LA County Museum of Art, a large US museum for art.
Regarding venues, The Walt Disney Concert Hall especially has concerts of the Los Angeles Philarmonic. The Hollywood Bowl seems to have an extensive list of scheduled events. Los Angeles has a lot of other venues, you can look up on ticketmaster.com or such sites for an artist you would hope to see.
Regarding clubs, I don't know much but I remember seeing a lot of people going out in Hollywood when I visited. I suppose you will have a relatively large choice.
Depending on where in Southern California you are going, you can also think think of more north San Francisco, famous for its numerous venues and various styles of "city life of twenty and thirty somethings normally entails".