We just stopped by the Seattle KOA to buy some propane, which was reasonably priced for the area. The folks behind the counter were very pleasant to work with.
While there, we also looked around at the campground. The campground was neat and tidy, though campsites are narrow. We saw campers of all sizes, plenty of cheerful kids, a nice playground, a nice pool, a pleasant-looking fenced off pet area, and those KOA trikes you can rent and ride about if you like. Yes, it is right off the highway and you'll probably get some noise. OTOH, if you're in the area to visit Seattle and see all the related sights, you could probably do worse.
There are other campgrounds in the area that we know of, though we've not used them. I'm sure they have their own issues, not the least of which will be highway noise. Here are a few possibilities, though none of them will give you that idyllic "National Park" type of experience.
- enjoys a "scenic" view of I-90, campers appear to be packed in like cordwood. Super-convenient location, if that's all that matters.
- right on Lake Sammamish, though it looks like folks there are probably dry-camping in the grass. Still, it is right on the lake and there's a swimming beach. They host lots of big family-oriented parties, so expect plenty of cheerful, noisy company during daylight hours.
- has some shade and it's fitted into a little triangle between I-90 and our local quarry operation. Definitely highway noise, possibly more quarry noise than you want on a weekday morning. They don't seem to operate the quarry on weekends, but I could be wrong. It seems pleasant enough and the location is convenient...but only you can decide whether the noise is tolerable.