@Geo: Cali is larger than Japan, but Japan has a population of approx. 127mil compared to our 37mil... WOO!!!
I recommend that you go to the Bay Area. "I'm from San Diego, so this is unbiased... HA HA!"
Both South and North California have plenty of things to do, but transportation wise, the Bay has a better transportation system than it does down here in SoCal... You literally need a vehicle to get from point A to B if you don't want to waste precious time on traveling down south, but the Bay has a subway/railway system that runs around the city without hassle, so there is the best bet if you want to be independent at times during your vacation...
The Bay has a lot of hipster-urban influences, so the younger crowd is pretty original to their personalities (slight droning... HA HA!)... SoCal, if you happen to stay around LA, the crowd is pretty much media driven and defensive, so you might run into the same person(ality) once, twice, thrice in the same day... HA! Just messing around dude, like everyone has mentioned, it solely depends on what you're looking to do...
How old are you by the way? 21 is the legal drinking age here in the states... HA HA!