For Washington, buy a Discover Pass
ahead of time. They're cheap, and allow free camping on state Department of Natural Resources (DNR) land, and free day use at state parks, but can be hard to find a vendor when you need one. DNR land includes some surprisingly good developed campsites
not far from the overcrowded/unavailable Olympic National Park campgrounds, if you're going that way, and if you aren't put off by logging activity.