West Fork Humptulips River
This is one of the longest temperate rainforest trails in Washngton State that's open to mountain bikes (~13 miles worth). If you like the Olympic Nat’l Park valleys like the Hoh, Quinault, etc, and want to *bike* a trail like that, put this one on your list. This is very much like those, but without the crowds.
However, maybe one of the reasons it doesn't get much traffic is the inconvenient fact that, over the course of its roughly 13 miles, the trail fords the river about 20 or 25 times. Most years, the only feasible/sensible time to ride the trail is late summer and early fall, when stream flows are lowest. On 9/6/2017, the USGS Humptulips gauge at US-101 showed 65 CFS, and most of the fordings on this trail were very doable, about a foot deep.
Some years this trail gets more maintenance than others; check the Olympic Nat’l Forest website trails page, or WTA, etc, (and of course this page) for recent reports.
More info here, on the ONF site: https://www.fs.usda.gov/recarea/olympic/recarea/?recid=47779
Expect some areas of brushiness and some areas with lots of roots and uneven terrain; this trail is mostly maintained for hikers/horses. Some sections are really fast and flowy though.
Also, this "trail" fords the river on average about twice per mile, and this includes some sections of walking along the cobbles of gravel bars (again, best only to go in late summer or early fall).
Unless you go mid-week, expect to see horses; make sure to get off your bike and be good to the horses and their riders; they're the ony reason this trail gets much maintenance!