How about pipelines and substations? Ports? If they can snuggle a drone in they might even go for something like a refinery. Not sure how the security around oil wells looks but hitting a couple of those might also be worthwhile.

Sure, sowing abject terror is one thing but ideally you also want to make the occupation fiscally unsustainable. America has a bunch of very expensive locations that would fit that part of the equation nicely.