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OK, let's start with some basic parameters and then try to make the best use of whatever 'leftover' resources you have. The drive between Chicago and Seattle will take a minimum of four days each way. That's at a steady, but safe, pace of about 500 miles per day. With two days in Yellowstone, that means you're already used up to 10 days accounted for. Leaving a maximum of 5 days in Seattle. BUT, if you can only drive six hours per day, and your wife can't spell you, then you're looking at seven days each way and you've already used up 14 of your 15 days just making the drive and the whole trip would be fairly pointless (only one day to split between Yellowstone AND Seattle. NB: Time estimates from mapping programs are a fantasy, the drive will ALWAYS take longer than they predict. So for the rest of this post, I'm going to assume that you can make good 400 miles/day as a compromise - That leaves you two days in Yellowstone and three in Seattle. On the other hand, only covering 400 miles/day leaves you plenty of time each day to make stops
en route.
The cost of a car rental is almost always cheaper to do as a round-trip rather than two one-ways. A single rental, returning to the same spot in Chicago, would also allow you more flexibility. You won't have to waste time making two trips to a rental agency in Seattle and you can make last minute changes to your itinerary such as staying an extra day somewhere on the spur of the moment rather than having being tied to a specific time to drop off the first and pick up the second one. You'll also have a car at your disposal in Seattle rather than relying on public transit or Uber.
I have never had to have a visa to enter Canada. Indeed when I lived near the border in Maine, I would often just walk across the border with my dog and merely wave at customs officials on both sides. However I was once denied entry into Canada, but that's another story and it did not involve having a visa. You should, however keep abreast of
any COVID protocols. For further questions, there are Canadian consulates in both Chicago and Seattle. Be sure to check with your rental car company to make sure that you can take the car into Canada. If you plan to go to Point Roberts WA, you'll need to check with your rental agency AND the Canadian government just to be sure that's OK - I've had a problem in the past with just that.
So, with those basics in mind, what else would I recommend? You've got several days to do more/other stuff on the cross-country portions of your trip. My first recommendation would be to use different routes east- and west-bound to maximize the number of sites you can visit. In one direction I-90 would provide access to Yellowstone and the half dozen or so unique venues around Rapid City SD. In the other direction, I-80/I-84/i-82/I-90 would take you by way of the old Oregon Trail, Salt Lake City, and the Snake River Valley. The second route is only 140 miles longer than the first and over five days of driving would barely be noticeable.
So, at this point, that's about t he best advice I can give you. Once you make a final decision on those basic parameters, particularly the number of miles you can cover in a day, we can get down to the brass tacks of where to take you overnight stops and what specific attractions might appeal to you and your family.
AZbuck