The Sierra Nevada range is beautiful country, every time I go up to Lake Tahoe I don't want to leave. But if you've come this far you need to go see the Northern CA coastline. I'd recommend heading over towards Mendocino, possibly passing through the Napa valley wine country and maybe cruising down to San Francisco to do the typical tourist thing. The coast is beautifully rugged country.
The Crater Lake suggestion above is also a good one, but it all depends on time you have, and distance you want to cover.
Enjoy your stay. California certainly has its "issues" with goofy laws and politics, but we are not short on scenic drives.