Leaving Missoula on Sunday, flying out of SeaTac on Tuesday, where should we stop? What do we need to see? Really don’t want to drive too far off the main freeway. We like seeing pretty things, but no necessarily out in the middle of nowhere. This is our first (and probably only) time out in this area.
Driving from Missoula to SeaTac airport in July - what should I see (w/ easy access to main freeway)
I have driven I-90 between Spokane and the Seattle area half a dozen times. There is nothing between Issaquah and Spokane. Between Issaquah and Seattle there are a few shopping options. Also, to make the trip easier, don’t take 90 into Seattle. Head south on 405 towards Renton. That will become Highway 518 and take your right to SeaTac.
I’ve never seen it, but I’ve had a weird fascination with the Scablands. Here is some info. It looks like it is a side trip just past Spokane, but may be of interest. I’ll let others who have seen it give more info.
I would drive to Yakima on Sunday and spend the night there. Then spend Monday hitting the wineries around there. Head to SEA on Monday night or Tuesday morning depending on how the flight times work out. Yakima has an excellent visitor center just off I-82.
I know you don’t want to leave I-90, but if it were me with the time to take, I would take Highway 12 nearly the whole way. You’ll go over Lolo pass, follow the beautiful Lochsa river and then up the Clearwater. You’ll hit Lewiston/Clarkston (a lot of this portion is the route of Lewis & Clark- hence the names) and then go through Dayton and Walla Walla (wine country). You’ll go through serious ag/farm/wine country in the Columbia Basin (Tri-Cities, Prosser, Yakima) and then get back in the mountains going over White Pass (watch out for Rainier…it’ll be bigger than life). Once you get on the west side of the Cascades, head north. This entire route is best taken by motorcycle. 
If you’ll be sticking to I-90, you’ll go over Lookout and 4th of July pass before you get to Coeur d’Alene which is definitely worth a visit. You’ll also go through Spokane which also has a nice downtown and park (Riverfront).
I guess it really depends on what interests you.
Suqualime Fallsis fright off I-90.
Snoqualmie Falls is as well. 
If you went on 2 rather than I-90 it’s right there.
The geology of Eastern Washington is crazy, if you’re interested in that sort of thing.
I 1000% agree with Racer72 that 405 from I-90 is by far the best way to get to SeaTac. If you can, plan to go between 9:30 a.m. and 2:00 p.m. or after 7:30 p.m. or you’ll get stuck in a traffic mess. Keep your phone on the traffic map regardless.
We went to Snoqualmie Falls last year and there were so many people! The parking situation isn’t the greatest, either. As an alternative, Rattlesnake Lake is very pretty and has cool rain drums and a nature center. You can take as long a walk as you want, or none at all.
North Bend (the outlets exit) is a good place to stop for your final approach to SeaTac (heh heh). It’s about an hour and a half to two hours away, and you can get gas, something to eat, and visit the restroom. I don’t recall there being anything fancy food-wise, but North Bend will get the job done. Issaquah will, too, but it’s busier and will take more navigation and frustration.
Good luck!
Too bad you won’t be traveling over the 4th. You’ll just miss the celebration at the town with my all-time favorite name, George, Washington, which features the world’s largest cherry pie and is just off I-90. 
You definitely should stop at Snoqualmie Falls. Spectacular.
I remember going decades ago before there was even a parking lot across the street.
That was the last time I’d been there, too, and I couldn’t believe the difference. So many tourists, a traffic jam, and difficulty parking. It was nice to see the Falls again, but I don’t need to go again for a long time.
Grandfaher Cuts Loose the Ponies is worth a quick stopover and it’s right off the freeway just before the Columbia River.
Look out for cougars in North Bend!
If you pass through Coeur d’Alene and have any interest in Western culture/artifacts, pay a visit to Cisco’s Gallery. It’s not a museum, but it might as well be–there are so many cool things there you could spend an hour wandering around the place.