Hi,
There's camping on the beach at Navarro River SP (Hwy 1 just south of Hwy 128).
You can stay at the parking lot across from Van Damme for one night only I believe, but there's always space there.
Along Hwy 20 - east of Ft Bragg - there are a couple smaller less developed campgrounds. It's part of the Jackson Demonstration Forest and there are several old logging roads through that area. I've only explored a little but it might be worth looking into more.
East of Caspar (mid-way between Mendocino and Ft Bragg) - at the end of Fern Creek Road - there is a whole network of old logging roads and I've seen people camping in vehicles up there (it's becoming a popular MTB area).
The campground overlooking the ocean in Westport is beautiful of course.
And then there is the dispersed camping area at Usal Beach which is amazing - provided you don't go on a weekend (it can be rowdy up there on the weekend). That area is 5 mi north on Usal Road from Hwy 1 (north of Westport). It's a dirt road, bumpy is places but drivable in any type car really - you see people go out there in Hondas...
I was up in the area last month and it was as pretty as I've ever seen Mendocino.
Have fun!