One that quickly comes to mind is where we went for the first CalX, which was down in Big Sur. One of the things that sets it apart from other trail areas is that it's right on the coast, so you can't beat the scenery. The Old Coast Road is great, especially for us urbanites. If you're interested in more of these coastal trails, I suggest you try and find "California Coastal Byways" by Tony Huegel. You might find it at bookstores, but you can get it easily online from Amazon or Barnes & Noble, or 4x4books.com.
To get a feeling of what we did at CalX, you can check out SCCX's page for it at
http://www.sccxterra.com/calx.html. You can also check out what we did last year at CalX2 which took place in and around Sequoia National Forest -
http://www.sccxterra.com/calx2/calx2trip.html. The annual CalX gatherings are always 2WD friendly. (BTW, CalX3 is coming up in a couple weeks down at Pismo.)