I'd recomend yosemite, but be prepared for crowds. I won't go any more just because it's so overrun, but everyone should see it once. However, I've never been to lake tahoe, so...

In the city there's plenty to do depending on what kind of person you are... First off, don't rent a car for your days in san francisco. Between Bart, Muni, and the busses you can get anywhere you want. Take a taxi if you really have to, it'll be cheaper than parking when you get there...

Look for some nice restaurants to try. They don't have to be expensive, but spend some money on some of the ones that have good reviews... The Metreon is sony's idea of a movie theater, and I'd recomend catching something that won't make it over the pond for a few months one night. (Stadum seating rocks my world...) Walk through china town and north beach one afternoon, as they're close together and far better to see on foot. The exploratorium is fun, but very geeky.

Northern California wise, I'd say go to either Mendocino or Monterey. (Disclaimer, I'm from monterey). If you want to follow tony's adivce and go to san simeon, chose monterey as it's on the way... Monterey is a tourist trap in a lot of ways, but it's got some great restaurants, a lage aquarium, and plenty of nice Northern California beaches. NorCal and SoCal beaches are compleetly different experiences... we've got room and walk on ours and enjoy them, but theyr'e not the see-and-be-seen kind of places...

There was a thread a few months ago which I think covered most of this, you can probably dig it up...

Matthew