I am a bit of a beer snob and when in San Fran want to try out some great brews (like Pliny the Elder) that are rarely seen in the Philly area. Any recommendations for top notch beers that are easily accessible in California, preferably on tap?
If you have time, Russian River Brewing (home of the Pliny family) is the place to go. They have some great and unique beers in addition to Pliny. Ridiculously crowded though. Earlier in the day is okay (they open at 11). Their food is mediocre, but beers are amazing and at an incredible price compared to what you pay in the store.
Lagunitas, as mentioned, is a lot of fun. It's in an unattractive industrial area, but the brewery itself is great, and the food's good. Tours are free and happen a few times a day--expect a lot of discussion about marijuana use, what happens when you dump yeast in a septic system, and, if you're lucky, a particularly surly tour guide will heckle the drunker tour group members.
If you're in downtown Healdsburg, Bear Republic is worth a trip. Probably not worth a special stop, but the beer's good and the downtown is really lovely.
San Francisco has a few breweries--Anchor is worth the trip for the history. 21st Amendment and Rogue are fine, but not worth an extra trip. For non-breweries with great beers, I'd recommend "La Trappe" and "City Beer Store".
Someone mentioned hiking at Robert Louis Stevenson State Park. Great hike, but long. If you're looking for something easier and shorter in the Napa area, Bothe-Napa State Park is lush and lovely. Skyline Wilderness is just outside of the Napa city proper--it's usually pretty empty and a nice escape. Between Napa and Sonoma Valleys, the State Parks of Sugarloaf Ridge and Annadel both offer pleasant hiking. At Sugarloaf, you can get up to a hill and catch a view. Annadel, just outside Santa Rosa, is a pleasant (assuming you're sticking to trails Mtn Bikers aren't allowed on!) oak woodland with a couple lakes scattered around.
Muir Woods is beautiful a common spot for tourists, being the closest spot to SF for redwoods. If you're already heading to Monterey from SF, you can stop somewhere along the way for a nice hike, depending on your route. Henry Cowell Redwoods, Big Basin, and Castle Rock are all quite lovely.
An inconvenience is only an adventure wrongly considered; an adventure is an inconvenience rightly considered. -- GK Chesterton