I could easily spend two weeks just exploring the coves and back country areas around Lake Tahoe, but Southwest Dave has provided many days of day trips. Although you have wisely cut out the cross country portion of your trip if you did all of the ones he has suggested and threw in San Francisco.... you would again be spending most of your time in the car. But if you wanted a couple more overnighters... Redwood, Lassen & Yosemite National Parks are all possible. Closer to Lake Tahoe, spend a day in Virginia City -- great bars and nice western heritage displays. If you want to drive on one of my favorite roads in the Sierras, make a day trip and drive south to western slope and then follow CA-4 over Ebbets Pass (there is a magical lake at the pass). When you reach CA-89 you can either descend the thrilling road over Monitor Pass or swing north and stop by the hot springs at Markleville.
Mark