Another must-see for August that is a little more on your route is Ape Caves. If you take the southern route to StHelens, I think it's the 503, and go through a town called Cougar, you'll see the signs. The caves are actually lava tubes from pre-historic volcanic activity. The good part is that it's a beautiful hike to get to the caves, and once you get inside, it's chilly.

Of COURSE this east-sider is going to go west for you, so you just do what you want to and have FUN.

~JE