From ATVFan.com :

East Fort Rock!    
by raptor8 on 12/28/2003
Pros: Total variety of terrain, beautiful mountain vistas, marked trails, campgrounds with potties, campsites, fire rings, tables, BBqs, unloading ramps and gravel roads, good maps, cant get crowded. OLD GROWTH ponderosa forest----bring your camera! the campgrounds are THE NICEST I have ever used. absolutely beautiful. NO FEES. Part of "your Oregon ATV funds at work." This area is Oregon's best kept secret. PLEASE enjoy it and don't screw it up - have a great time! 110,000 acres...larger than some STATES hehe... just try and get bored here!
Cons: grooming could be better. Bring your own water since the climate does not allow water taps at most camping areas. closures for 'fire danger' are stupid since spark arresters are required...but that only happens in high summer and not every year. be ready for 4000' to 6000' altitude, it gets cold EVERY night.

No mattter what you like to ride, you'll find it at East Fort Rock.
It is the largest USFS designated riding area in the lower 48, and a lot of ppl dont know about it. There is desert, to mountains, to forest - take yer pic. the campgrounds do not have trash haul or water, but do have fire rings, tables, gravel roads, signed and marked trails, designated sites for groups and permanent potties and they are free.! AND you are only 25 miles from town. Cell phone service is spotty, but there are always rangers around with satellite phones, etc. Spark arresters are required!
http://www.fs.fed.us/r6/centraloregon/recreation/cohvops/index.shtml

here's the site. plus, there's more info in 'where to ride'. let me know if you want to make the trip... it's a great time! most of the pics i've taken are out there.