Washington state has some truly stunning territory and great wheel'n areas. While the trail systems on the western side of the Cascades are closer to home for most of us and still a lot of fun, they don't have the feeling of being "out there" that the large ORV areas do, such as Manastash Ridge (access via I-90/Ellensburg to the north, Hwy 410 to the south, or Naches Trail to the west) or Rimrock (off Hwy 12).
Rimrock, being about 3.5 hrs from Seattle, is not as well known as the other areas. Given the travel time to and from I usually only get down there about once a year myself. And it only makes sense to go if you can take off several days from work to make the time worth while. A 4 day trip to Rimrock would be an ideal minimum amount of time (half a day to get down there and settled in, half a day to come home, and two solid days of wheel'n and camping).
Manastash is a bit "closer to home" being only about 2 hrs from Seattle via I-90. It's even closer if you start on the Naches Trail which is near Greenwater, WA. That route takes longer to get to the Manastash Ridge area (aka Naches Basin) but you're on a 4x4 trail for much of the journey.
Both Rimrock and Manastash offer huge swaths of territory and LOTS of 4x4 trail miles. In terms of trail milage Rimrock has a bit more miles at close to 100 miles of 4x4 trails. Manastash is around 90 miles. But Manastash is spread out over 115 square miles of territory. Rimrock is over a big territory, too but not nearly as extensive. In comparison, Evan's Creek has myabe 16 miles of trail, Reiter about 18 miles, Walker is in the same range, ditto for Elbe. And these western trail networks are good enough to keep you entertained for a day.
Trails tend to be dry and traction often isn't a major issue, even for open diffs during the summer and early fall. It's dusty and rocky with steep climbs. Obstacles aren't anything that'll stop a mildy prepped Jeep. But there are areas, such as Funnyrocks / Moonrocks, where you'll want a REALLY built rig if you want to REALLY play. The good news is if you want you can also be just a spectator as there are plenty of ways around the tricky stuff.
Rimrock is the one area I would offer a warning on. Terrain tends to be VERY steep with LONG climbs or descents. As such, make sure brakes and cooling systems are in top notch condition. Because of the severity of terrain I'd only recommend this for a summer wheel'n place. No way I'd want to be on these trails in rain, let alone snow.
Liberty is another trail area NE of Cle Elum, around the town of Liberty, WA with trails on either side of Hwy 97. I'm not super familiar with the "good" trails in this area. What I have run on is deceptively "easy". Gently rolling tails running up and down through the woods. But at the slightest sign of moisture the powdery clay trails turn into snot slick runs where even lockers are mostly useless.
If you want to see what Manastash looks like check out my video here:
In all three areas, Manstash, Rimrock, and Liberty, there is AMPLE camping all over the place. Both in terms of established camp grounds (some with toilets, some RV accessible) as well as disbursed camping out on the trails themselves.