There is very little rain in August. August is very hot inland. Sacramento is typically over 100 degrees. The coast is cooler as are the High Sierras. You can get a "Great Basin High" forming at that time of year blowing hot air of the deserts and the whole state will have a heat wave. The San Francisco area is almost always cool due to the coastal fog. As Mark Twain wrote "The coldest winter I ever spent was a summer in San Francisco"
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Central California might be a good compromise. Stay in San Simeon, relax on a beach, visit Hearst Castle. then head to Yosemite and Lake Tahoe. From Tahoe, take US 50 to San Francisco.
http://www.hearstcastle.org/
http://www.sansimeonsbest.com/
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Yosemite is one of the most unique places on earth. It's one of those places you must see before you die.
http://www.nps.gov/Yose/index.htm
If you want stunning vistas, Yosemite is the place. Glacier Point is breathtaking.
http://www.yosemitefun.com/glacier_point.htm
If you want to see big trees, there are groves of Giant Sequoias in Yosemite http://jrabold.net/yosemite/intro4seq.htm
After you visit the Yosemite valley and Glacier Point, drive across the park on CA 120. It takes you to the east entrance to Lee Vining near Mono Lake. The lake is in a caldera of an extinct volcano. CA 120 crosses Tioga Pass, the highest vehicle pass in CA. You climb thousands of feet on a mountain road and reach the Alpine meadows of Yosemite. From there you can take US 395 north to South Lake Tahoe
http://maps.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=mono+lake&sll=37.88786,-119.281311&sspn=1.604034,1.790771&gl=us&ie=UTF8&ll=37.956109,-119.215393&spn=0.774205,0.895386&z
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Even if you don't go to Yosemite, if you stay in the the Bay Area or central coast, head to South Lake Tahoe for a few days. The casino hotels have inexpensive rooms and great food.
http://www.bluelaketahoe.com/
The Sunday brunch at Harrah's Forest Buffet is one of the best I've every been to. It's on the top floor and has a beautiful view of the surrounding mountains and lake.
http://www.harrahslaketahoe.com/casinos/harrahs-lake-tahoe/restaurants-dining/forest-buffet-detail.html
The Vikingsholm Home & Castle is an interesting tour.
http://www.vikingsholm.com/tours.html
Visit Ehrman Mansion
http://www.tahoecountry.com/oldtimetahoe/ehrman.html
Hike to Eagle Falls
http://www.tripwiser.com/trip_thing_to_do-Eagle_Falls_Hike_South_Lake_Tahoe_California_USA?itiNodeId=8ad198040d86fd44010d87b2a5d87727&eType=activity&tripEleTabName=details&fromSearch=true
Heavenly has a gondola ride.
http://www.skiheavenly.com/lake-tahoe/lake-tahoe.aspx
Take a riverboat cruise and dinner on the lake
http://www.zephyrcove.com/cruises/index.cfm
Sutter's Mill is day trip down U.S. 50. It's also a nice drive along the American River. It passes through Strawberry CA. which has a lodge that was a stagecoach stop.
http://www.coloma.com/
http://www.strawberrylodge.com/
Virginia City is an interesting day trip from Tahoe. Made famous by the TV show Bonanza, it was the center of the Comstock Lode, the biggest silver strike in history. They have an old steam locomotive mining railroad you can ride and you can tour the actual mines. The city looks like it did in the late 1800's, complete with wooden sidewalks, saloons, gambling halls, and boot hill.
http://www.virginiacity-nv.org/info.aspx
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Sequoia & Kings Canyon National Park is another great spot in the summer
http://www.nps.gov/seki/index.htm
There are many trails that connect the Sequoia groves.
http://www.sequoia.national-park.com/hike.htm
Besides the Sequoias, there is a rare marble cave in the park (most caverns are limestone). Crystal Cave is accessed by a picturesque half-mile trail that leads to the Merced river valley.
http://www.sequoiahistory.org/cave/cave.htm
Moro Rock is a great hike. You climb nearly 400 stone steps to the top of the barren rock (6,725-foot elevation)
http://kevingong.com/Hiking/MoroRock.html
http://en.wikipedia.org/wiki/Moro_Rock
Enter the park from the south, CA Route 198. It doesn't allow RV's or trailers so there is less traffic. It's a mountain road that climbs thousands of feet, has great hairpin turns and the views are spectacular.
http://maps.google.com/maps?q=merced+map&ie=UTF8&split=0&gl=us&ei=CGyMSt3lMIiksgPG5oi4CQ&ll=36.486453,-118.875504&spn=0.422878,0.447693&z=11
There is lodging at Three Rivers, CA and it is close to the entrance.
http://maps.google.com/maps?sourceid=navclient&rlz=1T4HPIA_enUS314US314&um=1&ie=UTF-8&q=three+rivers+ca+lodging&fb=1&split=1&gl=us&view=text&ei=9m2MStz2LIWQsgP137S1CQ&sa=X&oi=local_group&ct=more-results&resnum=1
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BTW there is very little to see or do between San Francisco and Oregon. It is very hot on I-5 in August.