You can park and camp on most national forest, WDNR and fish and wildlife lands. This map has the filters set to highlight all the campgrounds with camping cabins or Yurts. Click on any point to get on links to the reservation page. https://mapswa.com/campwa/#8.00/47.119941/-122.014616?filters=true&recreation=CC I probably would not go all the way up around the Olympic peninsula which is a long haul. But after Seattle, you could take the ferry from Edmonds to Bremerton and then drive along hood Canal and across to the ocean. If you want a taste of the rainforest head up to Lake Quinault. And then maybe take 101 back along the Washington Coast. (The Washington beach is cold but really beautiful.)