The trail starts out loose, off-camber, and overgrown, but it gets better once you get up the first part. It's steep but short, and you are rewarded with 360-degree views and maybe an open lookout.
To get there: Take 90 to SR 903 north, then left on FS 4308. Follow the sign for Knox Creek. At some point, there is a fork in the road; go right onto the rougher road. The trail starts a bit further down. There are not many parking spots. No pass necessary.
There are four ways to get to Thorp Mountain: the Knox Creek Trail, the Thorp Creek
Trail, the Kachess Ridge Trail, and the No Name Ridge Trail. This route is actually a combination of Knox Creek, Kachess Ridge, and No Name Ridge.
The trail starts from the road and almost immediately starts to switchback. The trail here is quite overgrown, loose, and off-camber. It is exposed as well. At the first intersection, there is a nice lookout, but ultimately you want to go right. The trail (you are now on Kachess Ridge) is much nicer now in the trees. It goes downhill for a little ways, but you'll have to regain this elevation. The trail becomes steep again and more exposed. After that, it will level out, and you'll reach another intersection; go left toward Thorp Mountain. Now it gets steep again.
Soon you'll see the lookout, and this is the end. You can see Little Kachess
Lake and Thorp Lake
. Not sure about Rainier, as it was cloudy. There is a trail that goes past the lookout, but stops shortly afterward. The lookout was not open.