MNF is a seriously good time! I'd recommend, Lake Pillsbury and Letts Lake if you're looking for spots by water.
Pillsbury is the largest lake in the forest and has some good easy camp sites along the northern edge; Navy Camp, Oak Flat, and a few at Sunset Point. I've only camped at Oak Flat there which is the largest of these, but it should be pretty empty at this time of year.
Letts Lake might be easier for you to access from the Sacramento area through Stony Ford. I think Letts is the most beautiful in MNF (although I believe all of them are man made). There's camping along the eastern and southern edges of the lake but I think only the sites along the southern edge will be open this time of year. This is a great place to bring a canoe or some other motor-less craft.
MNF is chock full of fire roads and trails and is the only National Forest that does not have a paved road running all the way though it. You will be on dirt roads 95% of the time in there. Many roads are pretty smooth and well maintained (especially M1 which is even paved in some stretches). Smaller roads will be more like trails and then there are much smaller dirt bike and ATV trails (not open to street legal vehicles). There are also bazillions of miles of hiking trails and even equestrian trails and camp sites. Have a look at google maps, you'll see that its a huge tangle of roads to explore. I've been going here for about 15 years and I think I've only seen about 5% of it.
Here's a link to order maps for MNF. I have both the folding map and the atlas, but I'd get the folding map if you only want to get one of them:
Folding map:
http://www.nationalforestmapstore.com/product-p/ca-9.htm
Atlas:
http://www.nationalforestmapstore.com/product-p/ca-70.htm
There's no substitute for paper maps in hand but here's a link to download a PDF map of MNF (17MB, too big to attach here), right click to save to your computer:
https://www.dropbox.com/s/v914cv76ig1sqxw/mendocino.pdf?dl=0
