Rainbow Falls State Park is a 139-acre camping park with 3,400 feet of freshwater shoreline on the Chehalis River. Situated in stands of old-growth forest, the park features a waterfall and a small fuchsia garden.
Sasha gave most of the important information about this hike. I've uploaded a picture of the park map so that others can see what she's talking about. The bridge going over the river does not exist.
As for the trail, it's a pretty nice one. I saw no other hikers on the trail and even some of the trails had fallen trees and broken foot-bridges. This is something that I actually find charming. It's nice to hike on a trail that doesn't see much use. The small creeks that criss-cross the trails are pretty cool, too.
Beware, there are trails across from the park and the bridge to cross over is non-existent. You have to cross over the highway to get to the trails. When you do get to the trails, you'll see that there are actually three little trails so the miles stated are for the combined trails. Aside from having to park on the highway after finding out at the bridge was no longer there, it was an okay, short and easy hike. But not something I'd travel again to do.