My maps are out in the car and I'm still in my jammies so I'm not going to be able to look for highway designations but I will suggest that you avoid if possible I-5 from Olympia to Everett. Driving through Seattle is really tough anytime, far worse that most of L.A.
You can visit Cascades then drop down I-5 a way to catch the ferry over to Pt. Townsend then on to Olympic NP and then down and over to Olympia and back up to Rainier. 101 basically circles the Penn. but Kala Loch(Clay Lock) a nice campground is on the West side as is Hoh Rain forest(Mick Dodge). Forks is on 101, it is where Twilight was filmed. There are campgrounds all over the Penn. You would do best by Googling them.
Remember Rain Forest and Hurricane Ridge can mean some foul weather. Only problem the weather presents to me is walking the dogie. Google Washington St Ferries for routes and times.
