I rode the trails at O'Grady Park today. O'Grady is a King County Park on the Green River just down from Flaming Geyser state park. Mostly downhill and cross country riding. They have about 6 miles of trail but you can ride out to the river on three separate trails and hang out on a beach. I took the kids out so it was a slow speed ride. The trails are currently being brushed out by the County folks, so watch the corners. I asked the county guy to send me an image of the map so I could pass it along. (See attached).
This is primarily horse trails but it sounds like its mostly being used by hikers.
Have fun enjoy the ride.