Californian expat here, I know good places to visit and when regarding the central cost, the north coast, the central and northern Sierras, and the desert regions.
In order starting at the beginning of the year:
January:
Generally you want to plan any trips to the Mojave areas, like Death Valley, Red Rock Canyon, Joshua Tree etc for January because it's kinda right in between the rains. However, make a note that some roads may be washed out because of prior rains and that various land management services have not cleared out anything other than the blacktop. In DV I highly suggest visiting Scotty's Castle, the borax works, ubehebe crater and badwater basin. The Racetrack is neat and all, but it's a long drive along a terrible road. Bring 4wd (not AWD) and patience for the smoothest ride but it can be managed in a 2wd vehicle. Pack for relatively cold weather, as it's january in the desert, temperatures can dip below freezing.
February, March:
Best skiiing and snowboarding time. Plan your trips up to the various ski resorts in california for this time period.
April, May: Now is the time of the spring festivals in the central San Joaquin Valley, small towns, fair food etc.
June, July, August:
Coast or mountains, you don't want to be in the valley right now. Temperatures get up to 115 sometimes. for the Central Coast, I suggest Monterrey, Avila, Cambria and Pismo. North coast I suggest Fort Bragg.
Activities range from surfing, boating, fishing, snorkeling, searching out rare moonstones in the beach, abalone diving and clamming.
For the sierras I suggest: The shasta-trinity national forest, Sierra National forest, Yosemite National Park, Kings Canyon Nat. Park. Activities include, fishing, camping, hiking, climbing and some of the best stargazing. Also various locations will have 4th of july celebrations (though for the best one I suggest a little town in idaho by the name of Crouch.. it's basically a block party where everyone brings a small truckload of fireworks and the private party outstip's the local fireworks show by a good margin. bring ear-pro and eye-pro)
Additionally the California State Fair is in July... however that's in Sacramento, very humid, very hot. Go at your own risk.
September:
Generally something of a slow month, not much really that I've done and wandered about on this month.
October:
Summer fair season is coming to a close, The Big Fresno Fair is one to check out though. Beware, it can still be relatively hot at this time of year.
November: I don't do much during this point in time, too busy with other stuff normally.
December: the only thing I generally try to do during this month, aside from christmas and catching up with family is make a trip to the nation's christmas tree in the general grant grove in the Sequoia Kings National Park.
