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Coachella is in April out in the Californian desert but I think its sold out already. San Francisco is fun but now that cd stores are pretty much obsolete I dont know if haight ashbury is as cool as it was a few years ago. Pretty city though in some parts.
I'm in San Diego which is Southern California a really good place for beaches, also great food and a downtown area that's like a mini San Francisco.
Unless you plan to go to Disneyland, I would stay away from the traffic, smog and strictly city area known as L.A. All 3 areas can have bad traffic somehow L.A. Seems by far the worst to me.
All 3 areas have decent nightlife, clubs, bars etc. But festivals are held only every so often and jamband type festivals are probably going to be mainly around the San Francisco area. reggae/punk/alternative are more common down here, though there are some small hippy populations near the beach. There are other areas like Humboldt and Santa Cruz that have hippy populations but they're a bit more low key, not much tourist attraction stuff.
Bring SweetBlasphemy if you come here to San Diego.