Don't scratch North Cascades! Favorite mountains!
SR 20 over the cascades is the prettiest road I've ever been on, so make sure you do that. I might be biased because we used to spend summers there when I was a kid, but the Twisp/Winthrop/Mazama area is pretty great, and SR 20 sets you down right in the middle of it. The Methow has some great swimming holes, and hikes up the Twisp River drainage are both seldom-used and beautiful. Reynolds Pass hike is my favorite, but might not be doable for a 6yo. Lookout Mountain is also a favorite, and is probably more doable with a kid. If you have a good car, you could also take the drive up to Hart's Pass
and camp right near treeline, which would mean you wouldn't have to conquer a ton of uphill hiking before getting to good views. Poplar Flat
is also a nice campground and doesn't require the rough roads Hart's Pass requires. It's near a nice quiet section of river. Twisp has a burrito place with a vegan option that's pretty good (rice, beans, GIANT roasted poblano, etc) + a nice farmer's market + a health food store that generally has a vegan lunch option, and has all the other vegan staples you'll need.
The Maple Pass Loop
is right off of 20 and is probably the best quick access to classic North Cascades scenery I've ever seem.