Not Much in That Time Frame
Welcome to the RoadTrip America forums!
Well, you've already got a start. They'll be going from Kansas City (I'm assuming from your user name) to Seattle. By the most direct route, which will take them by Mount Rushmore, that is going to be a little more than 1,900 miles by the time you add in the extra miles to get to interesting places along the way. That means 4 days of serious driving, not the 27 hours the on-line mapping routines will promise. Remember that those computer programs assume you always drive the speed limit, never get slowed down by traffic, never stop for gas, or to eat, or to go to the bathroom, or sightsee, or even sleep. Really, they need to plan on at least 4 days to make the drive plus any time that they will spend actually visiting places.
And there is so much to see on or near their route, that they may want to consider adding several days for the drive. Just around Mount Rushmore, there's also Badlands National Park, Wall Drug, Wind and Jewel Caves, the Crazy Horse Memorial, and Devils Tower. At various other points, there's the Corn Palace, the Little Bighorn Battlefield, and several scenic byways in northern Idaho around Coeur d'Alene. Without going too far out of their way, they could also include visits to Yellowstone National Park and Mount Ranier.
So the first thing they and you will need to do is to sit down and decide which of those places they'd like to see, and how much additional time they can budget for the trip in order to make such visits possible. Once those decisions are made, then we can help you find some reasonably sized towns that will make decent overnight locations and will have a choice of motels or other accommodations.
AZBuck
Great itinerary from Arizona Bob
If the sights he listed interest your parents, then they really can't go wrong with those suggestions.
I might suggest that they visit Grand Coulee Dam after Spokane and before going south through Kennewick to eastern Oregon. It's really amazing and the tours and laser light shows on the dam face at night are pretty cool.
And this area is so interesting, geologically speaking. This is where the great ocean of the ice age that covered Montana spilled through to the ocean when the ice melted, carving these unique coulees. This is the only place in the world where these structures exist. Although they do look similar to a few other places on the planet, those were formed by slow erosion processes while the coulees were formed over just a couple of days. I think the whole thing is fascinating. Dry Falls, just south of Grand Coulee Dam, is worth checking out as it was once the biggest waterfall in the world, dwarfing Niagara Falls.
Anyway, if this slight detour sounds interesting to your parents, I can give you some more details on how to get there, etc.
I also wonder if your parents wouldn't want to take a bit more time to do the route Arizona Bob suggested. I'm just thinking that their age combined with the fact that it sounds like they haven't done many roadtrips means that they might get real weary if they don't take some rest days here and there. They might want to plan at least one day a week where they lay over someplace nice just to get a break from the car. It could be someplace where they're in a little town that they can just explore on foot for a day and/or a place with a pools that is nice to lay next to and relax, or something else like that which gives them time to recharge their batteries. Just a thought.
I totally agree with Tony that Mt. St. Helens is a must. I go back about every two years or so, sometimes yearly, to see nature renew itself. I sure wish I'd taken a better photographic record of it all.
I usually go to Paradise area when I visit Mt. Rainier. I think there are things there that would still be fine for your parents to do. At the entrance at Longmire there is a nice museum and there is a nice, new visitor center at Paradise worth seeing. Plus the lovely Paradise Lodge, built in the 1930s as a WPA project, has been renovated and is nice to visit. At Longmire there used to be a short, flat interpretive trail that was interesting. I'm unsure if it's still there as that area got a lot of flooding a few years back but, if it's there, your dad should enjoy that. And up at the Paradise visitor center area, there are numerous very short walks to take you out to look upon the glaciers and more that you don't need to be in super-good shape to walk on. If I recall correctly, they are relatively flat with just modest elevation gains. Many of these are handi-capped accessible so they couldn't be too bad. And there are ranger-led walks from the visitor center on some of these so your folks can hear some interesting history/stories if they went on one.
Let us know if we can help you further.