I spent a lot of time out on the mountains off of I-90 with my kids (they are older though). Mt Si, Rattlesnake ridge, Lake olallie, snoqualmie falls, etc
Cost was outrageous though.
How many people were there when you were at those places? I hiked Mt. Si, and it was slightly more crowded than my local shopping mall on Black Friday (in theory - I went to the zoo this Black Friday with my kid and some friends).
I think part of the problem is that my wife grew up in Idaho (almost exactly where we live), and we met in New Mexico - where hiking and being outside generally involved not seeing people you didn't invite with you - and, as a result, insane amounts of local wildlife. I cannot tell you how many lizards, centipedes, roadrunners, and the like we saw while out and about, because we were out places that people weren't.
Hiking around Seattle? Rare to see much more than a camp robber (rather annoying species of bird). The animals had long left the trails.
I really enjoyed the trails that would wind through Bellevue connecting the parks and neighborhoods. Always was able to find some wild blackberries or go to the little beach near I-90 on the Bellevue side. Not too bad for outdoors entertainment, but I guess it doesn't even compare to Idaho or even the area near Issaquah and North Bend.
Trails winding through neighborhoods are a far cry from Jeep trails winding through the wilderness, where seeing another person is somewhat surprising, and a cause for conversation.
I can't speak to Issequah or North Bend - it took us forever to get anywhere, and after about 2 years, we honestly just gave up trying to find places outdoors. If you didn't get there at 6AM you couldn't find a parking spot at the trail head, it was absurdly expensive, and it was like being in a shopping mall, just outside.
I thought it was a miserable hellhole of a place to live, as did my wife. The money was nice, and the resume entries mean I'm set for life, but... eh. It was kind of a coin toss to move there in the first place (we were very happy where we were, I interviewed with a company on a whim, and they are a well regarded tech company that is useful on my resume). The experience up there was miserable, and the end result is that I'm now working for my former employer, remotely, part time, somewhere we love living.
Obviously plenty of people like Seattle. It's a great place to sit in traffic to stand in line to spend money (or, if you're doing it properly, apparently taking public transit or Uber to stand in line to spend money), but if you don't value those things, and can't get the things you value (stars, open space, silence) - it's just miserable.