My computer desktop wallpaper is this satellite image of the PNW centered on Washington and going up into BC. The Great Columbia Plain, or Plateau (even though it is surrounded by mountains) of Eastern WA is striking. Can clearly see how this flatter, more arid region extends into the Okanagan region in BC, with Kelowna and all.

And how most of BC is not like the Columbia Plateau. Besides the Okanagan region there are pockets that look somewhat similar in the BC Interior, up the Fraser Canyon, Lillooet area, the Chilcotin plateaus, but mostly that region is more rugged and constrained in valleys.

Other things I like about this image—BC's Coast Mountains are dense and right up against the ocean, leaving very little land similar to Western WA, except Vancouver Island and the Lower Mainland. Though there's nothing in WA quite like the lower Fraser River and delta, which dominate the Lower Mainland. One could argue that the lower Columbia River is similar, and it is in some ways, but also different in a bunch of ways.

I also like how on this image you can see the network of large flood channels in eastern WA where the Missoula Flood waters rampaged, making lots of coulees and the Channeled Scablands. Also how clear the Willamette Valley in OR is, and the darker greens of Olympic National Park, compared to the patchy logging lands around it. Also how the Columbia River doesn't take the straighter path through the Columbia Plateau but skirts around the northern and western edges, going north and west of Banks Lake / Grand Coulee and Moses Lake, by Lake Chelan and south along the very edge of the Cascades. I think long long ago flood basalt lava filled the region and forced the Columbia to "go around" as best it could, making it take a rather circuitous and twisty route.

How different it would be if the US and UK had agreed to make the Columbia the border!

All that said, when my wife and I moved to Washington from the eastern US about 25 years ago one of the first longer road trips we took was up through BC to the Canadian Rockies, and we were very surprised by how arid and epic the landscapes of south-central BC are. The grandeur and also the dryness of the Fraser Canyon up near Lillooet really surprised us. And driving from Lillooet to Vancouver felt sorta like your description of I-90 in WA. The landscape went from dry arid steppe-like semi-desert shurbland to dry forests similar to the eastern slope of the Cascades in WA, to the super lush areas around Pemberton and southwards. Lots of geographic diversity up in BC too. And fjords galore! Not too many of those in the US outside of Alaska. More recently I've been learning a little about the far north of BC and things like Level Mountain, a vast volcanic complex with "unusual" landscapes.

edit: a coupel tpyos