You obviously saw The Grand Canyon. If you put that into your "top-5" list of things you've seen, then you'll want to add Yellowstone.
Also, when going from Boise to Seattle, make sure you get there or get back from there via the Columbia Gorge (better to do both (to Seattle on the Washington side of the Columbia River and back from Seattle on the Oregon side of the Columbia River) as it's a wasted trip going straight from Boise to Seattle through northern Washington as it's a night and day difference between the two options.
Here is a picture from the Washington side about 27 miles East of Vancouver, Washington. Another 1/2 mile is a road that takes you 4 miles up (to the 1800' level) to where my parents moved to while I was finishing High School. (They lived there 10 years before selling the place to some people who had a full-grown Bengal Tiger for a pet.)
This is an image from the Oregon side about 30 miles East of Portland. That's Mt. Adams in the background. There's a little rock (Beacon Rock) in the upper left of the picture. It's a nice easy climb that takes about 40 minutes up and 20 minutes down. (My girlfriend and I climbed it the 13th for fun and exercise.) That view is from the same spot on the Columbia as the picture taken from the Washington side where I said another half-mile and you take the road up to where my parents lived while I was in College.
This part of the gorge will look like this in July or August, but you won't get the rainbow, as it won't be raining...the only two months it doesn't rain at this section of the gorge (10 miles East of Portland in Corbett, Or.).:
Drive another 10 miles and you'll come to this (with several other falls in-between if you want to stop for any of them while you take the "Columbia Scenic Highway" through Corbett). You can hike to the top of the falls above the people on the bridge (it's about a 40-minute hike to the top and 20 minutes down):
Near those falls is another small one. You can swim in this one if you wish, though it may be only shallow enough to wade in during August. It's about an hour hike up to it through a forest trail. It's called Eagle Creek Punch Bowl.
Back to the Washington side, a nice place to visit in the Summer is The Maryhill Museum.
I honestly can't think of any reason to spend time in Portland or in Seattle, other than they are on the way to beautiful Vancouver, B.C. and other places. The Olympic Peninsula in NW Washington is a pure paradise of wonder. Driving along the Oregon Coast (all beaches are public, while even in July and August are not very inhabited). Just driving along that Coast you'll come across 100's of scenic stops.
There was a REDSZONE member many years ago that took a group of high school kids on a bus tour doing the route of the Columbia Gorge West to Portland, down the Willamette Valley as far south as the Wildlife Safari before heading West to the Beach before turning North and driving along the Pacific Ocean. You could take that route (stopping at the Astoria Column)
and continue North across a 4.5 mile floating bridge that spans the mouth of the Columbia River into Washington. From there you could make your way along the Washington Coast up into the Olympic Peninsula.
Also, this photo is of the South end of the Hood River Valley (70 miles east of Portland). You can see this by just going about 4 miles off of the main Highway (I-84) on the east end of Hood River. (if you want to know exactly where pm me). Those are spring flowers, so you won't see those, but the rest should still be green as those are farms below that stay irrigated and the evergreens stay, of course, ever green. We went here the weekend before (so we could get out of the rain in Portland, as it was dry and sunnier here only 70 miles away).
Another trip worthy of hiking for a couple of hours is Silver Falls State Park. The drive along old Hwy 99 South to Silver Falls is about 100 minutes in length from Corbett while being as beautiful as any Valley I've ever seen as it's almost all farms with rolling hills and the tree-lined Cascade Range to your left. The Falls have a route you can take where you can see 6 of them or all 10 of them. There are a few that are 130+ feet in height. The Columbia River used to run through there 15 million years ago. Here's one of the falls (you can stand behind it):
We did that June 6th.
There's a zillion more things I could tell you about which are nearly or perhaps just as much as inviting as these places. I'm sure others have some that are more so.