I camped at Mckerricher state park a few weeks ago, near Fort Bragg, and found a couple of good rides. One is along the coast, between Fort Bragg and Mckerricher. This is a paved, very flat trail. The next ride is in the Jackson State Demonstration Forest, near Mendocino. I started fairly high up the slope, along the Forest History trails. This was very steep, I wouldn't recommend it too highly, unless you want steep MTB action.
When I got to the lower portions of the trails, I had a lot of fun riding along the Big River. It was relatively flat, smooth soil with some gravel. I didn't go all the way along the trail, due to time, but it had to be a couple of miles long at least. This would be from the intersection of Road 700 & Road 730. I think Trailforks calls this the Big River Haul Road, and it looks like it goes all the way to Mendocino.
I found a site describing a new trail in this area with a bit of climbing, it's the Observatory Trail. Below are a couple of links that may be helpful.