As a former resident of California, still with some friends there (and back there in just over a month) I agree with most of what has been said, in particular:

• AVOID Tijuana. Even twenty years ago, when it was a lot less dangerous, it was just tacky and full of drunken Californian teenagers and various scammers trying to take advantage of them. Now it's far worse. What are you thinking?
• SF (please not "Frisco") is worth more than just a day trip. Spend a minimum of two nights there. In June you might even avoid the evil fog and get beautifully sunny (though cold) weather.

However, along with lancefan and Yahilwe I would agree that LA is a fascinating city, compared to which SF and San Diego are sleepy and socially dead. San Francisco has the same inhabitants it did forty years ago, but they moved on from nightclubs to leather bars and now to gay retirement homes. Friends who live there are B.O.R.E.D. and spend a lot of time away. But LA does take a lot of discovering and appreciation and although a fantastic place to live, or even visit if you're in the know, it's a bit tricky for a casual sightseer. If you have friends there to help show you around, that will help a lot.

As for San Francisco, it is incredibly beautiful - I think the second or third most beautiful natural setting of any world ciry I know about (behind Rio, and I can't quite decide between SF and Cape Town). If you have time go right to the top of the hill where MSRI is at Berkeley to get the most stunning view of the area. Great restaurants, plenty to enjoy walking around, but why o why go to the tacky tourist trap of Fisherman's Wharf? Yuck. What's the point of that? And well, a cable car is a cable car. I suppose I occassionally hop on one if I want to get to the top of a hill quickly, but it's hardly a destination in its own right.