Another vote for the North Yuba and the extension if you want a really long ride. It's 16 miles one way from Downieville to Rocky Rest campground. From Rocky Rest to Goodyears Bar it's mostly rolling singletrack along the river and it's a beautiful trail, about 16 miles out and back. From Goodyears Bar to Downieville is quite a bit more challenging ride, but after a big climb up the mountain there is some sweet downhill into town.
You can also drive up to Second Divide and park, then ride up that and do an out-and-back on part of Big Boulder, then come down Third Divide. At the bottom of third, just take the road back to your car. You'll still hit some downhill traffic, but it wont be as fast as you go up on second.