Auburn and Placerville, CA, 25 and 35 miles east of Sacramento, respectively, are both cities of moderate size and both are gateways to outstanding riding in the Sierra foothills (the famous Gold Country) and mountains. Think nice curves and plenty of twisties. It would be easy to incorporate Lake Tahoe into a days ride although I wouldn't recommend headquartering there for reasons of congestion and cost. Rides from either city have spectacular scenery and from a start at about 2,000 ft. elevation, could include mountain passes to well over 8,500ft., perhaps even over 9,000ft. Plenty of lodging and good food at both locations.
Interstate 5 provides fast transit from the north or south for those in a hurry to get to the area while there are fun two lane alternatives for those with more time to play. Riding up Hwy 49 from it's southern end is a destination ride in itself.
The central coast offers good possibilities too but would be a much longer haul for those from up north. The coast (as well as Tahoe) would also be more difficult if lodging and meal costs are a consideration as I expect they would be.
In planning rides for California, I'd suggest subscribing to Pashnit at www.pashnit.com. You can check out the site at no cost. If you check it out, be sure to try the free samples of some of the road sites. Check out Mosquito Ridge Road which begins about 25 miles out of Auburn in Foresthill. It has 600 turns in 36 miles. This site gives detailed info. and pictures on all the good bike roads in California. The lengthy descriptions would also cause caution about taking a group on some of the roads. The site's done by a rider and is worth the $25 just for armchair riding if nothing else. Those of us in the area would be happy to suggest good rides and I know a number of us would be happy to help in any way we could.
From my home in Sacramento, I am 25 minutes from rides that other folks take vacation time to enjoy. Check it out and any questions, just let me know. Bruce