For those traveling along I-90 near Snoqualmie pass there's a little known campground that's perfect for an overnight, day long, or several days, stop.
On the map it shows as the boat launch area at the NW end of lake Kecheless.
This lake is an irrigation lake and the camping area is under several feet of water by spring but drops rapidly until the water is well beyond the end of the boat launch ramp.
Pull in, grab a level spot (there are acres of them) and enjoy.
Take the Hyak Exit (East or Westbound). and then follow signs to boat launch.
I've driven past it for decades and never even gave it a thought to stop and camp there. Just spent 5 days there, watching my son play with R/C Rock Crawler "vehicles" along with about 100 of his "friends".
Some pic's of the area:
Son getting ready for the "race"
It definitely is a "Boondocking" area as there is NO water or even "Vault Toilets". It is about two miles from the "Iron Horse State Park" Hyak trailhead and there is water there but you may need a "Water Thief" if you need to hook up a hose.
You do need a Forest Service Pass but in the entire time I was there I saw no "enforcement" (I have a Lifetime Pass). The State Park requires a Discovery Pass but there's no camping allowed there.
Here's a link to Google Earth and the area at the "top" of the picture is almost all dry right now.
While it looks close to I-90, in reality its far enough away you can't even hear the traffic. Cars and trucks look like ants.