If you want more structured snowshoeing or x-country skiing, go to snoqualmie east when it opens. you can take the lifts up and it has tons of areas you can go to. I think the nordic tracks are all near there as well. It's only 45 mins from Seattle, so hard to beat.
Also, if you are more adventurous, I like to snowshoe behind Alpental. There is a trail, but it tends to be a great area to trailblaze, and there are usually other people around. I have carried my board on my back out there and had some amazing times.
I know there are tons of places along I-90 where people just park and have a blast. You can always just look for groups of cars, and that's likely what is happening there.
Good luck!
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