Seattle to Vancouver, BC
In Lynnwood, there's a Kirirom for Cambodian. Very nicely executed. Right off I-5.
In Everett, there's Pasteur for above-average pho with a selection of banh xeo, etc. Right off Highway 99.
Both of these should be pretty nice for lunch. Of course, there are many Korean places on Highway 99 around Shoreline and Edmonds, but that's even closer to Seattle. There's not too much past Everett until Bellingham. But by then you might as well go to Vancouver.
Everett has the Depot Smokehouse - great barbeque. Near the train station, obviously.
The Conway Pub is known for its fried oysters and burgers, and is easy to get to from the freeway.
Mount Vernon has Thai House, Poor Siamese, Pacioni's Pizzeria and the Porterhouse all available for lunch downtown, depending on your mood. We are also very well supplied with taco wagons here.
The Burlington Cafe has excellent organic/local food, and makes great sandwiches.
Unless you're going to the Lynden/Aldergrove crossing, you're going to be sitting in a long, long line at the border in your car. You might consider packing a lunch or grabbing a takeout sandwich (one that will survive a couple of unrefrigerated hours) from one of the aforementioned places in or close to Seattle. (Banh Xeo are great for this purpose, though I'd still throw them in a small cooler with a couple of cold bottles of water and an ice pack to keep the pickled veg from going too funky.)