I live in Ashland, OR and around here we just go on some of the Forest Service roads or  logging roads. There is a cool road that goes up to a lookout tower that is fun in the snow. Some times if they are no longer maintained they can be pretty muddy. I would have to say get a forest service or blm map. They show umimproved roads on them. You can get them at GI Joes. Try heading out Hwy 58 I saw on yahoo maps that it heads out towards crater lake there should be some service roads or logging road as you get into the  National Forrest. I have never really been out and about up there, so I dont really know anywhere specific, just some ideas though.
  I would deffiently stay on the roads though, you can get a big fine , I heard about one guy that got a $5000 fine for tearing up a field. I hope this helps

Have fun
Jacob