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East coast: DTW
West coast: PHX during the summer

Basically nobody wants to go on vacation to these cities (DTW year-round, PHX during the very hot May-September period), however they are a few hours drive from other hard-to-get-seats airports (ORD and LAX).

Seats do open up, but much easier to get 1 than 2 at a time, so you are probably best off looking at unloved destinations.