......come down to Washington west side of Cascades (highway 5) and cross over highway 20, the North Cascade highway, it is the second highest mountain highway in the country, second only to Going To The Sun which you will have an opportunity to go up in Montana at Glacier Nat. Park. North Cascade is stunningly beautiful, there is a nice easy walk to a lake at Rainy Pass for a drive break, also Liberty Bell mountain area has spectacular views. This does have grade, but it is a well made highway with gradual increases, I think it is worth it, we pulled our 13" Burro
over it several times with no problems and plan to take the Escape
You could also drive over highway 2 - also scenic and not the grade, but you'd have to go north a bit to Grand Coulee
If you drive highway 20 you will come down to Winthrop - Twist area, Winthrop is touristy but it's frontier town motif has charm especially for kids. You can continue on highway 20 over the next 'hills' to the Okanogan area, there are DNR campgrounds along Loup Loup summit area, of if Winthrop is as afar as go first day there is a state park there, Pearrygin Lake. There are also campgrounds as you come off North Cascade Highway before you get to Winthrop. Sounds like you might try to get to Grand Coulee first day, highway 20 will intersect 97 at Okanogan.
It would be a spectacular way to start your trip and if you go up Going To The Sun in Montana you will have driven the two highest paved roads in US on your vacation! By the way, we left our Burro
in the campground and drove up that one, much steeper and twisty than North Cascade Highway.
Highway 20 is a great way to cross Washington to and from Idaho, far more scenic than 90, if you are going to the Grand Coulee you will be heading south of it but if you have the time coming home I recommend it......or for another trip!