There are so many things to do here in the Northwest. Here's a list that I stole from someone else of things to do here in the northwest. I'm happy to give recommendations!
Twin Falls Hike
For a less touristed, very northwest hike with mossy trees to a nice waterfall, twin falls is definitely the better choice than Snoqualmie. Very mild hike, appropriate for all abilities!
The San Juans are a gorgeous place to visit, though dealing with the ferries can sometimes be more than a pain. I would suggest taking the Victoria Clipper straight from Seattle without a car to San Juan Island if you're only going to go for a day. Otherwise, you will drive 90 miles to Anacortes and take the ferries from there.
Located on the southern end of Vancouver Island in British Columbia, Victoria is a cute town to visit. The Victoria Clipper also has a high-speed boat from Seattle to Victoria so you can visit the town for the day.