I would get in touch with the Sierra Club. Here is the website for the Norther Cal Chapter-
http://motherlode.sierraclub.org/
They organize all kinds of outings and will likely know where almost every trail and campsite is in the area. Also, the people who work at REI are usually hikers and campers, so they often know where to go.
I would also look online for websites for Northern Cal. If you Google "northern california hiking trails" a lot of sites pop up.
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