When they pick their targets (historically) they want 3 seperate things. 1) Publicity: they want media coverage so that it not only creates fear but so that it helps to create recruits. 2) Economic: think of how much the 9/11 attacks directly cost and how much they cost indirectly. 3) Symbolism: every where in the world people knew the New York skyline before 9/11, now?
If you pick a terrorist attack and look at the targets, every time these 3 things apply. Now think of an American symbol; sports, hollidays, locations. Think of all the soft targets. To me, one of the worst would be a bio attack such as this on Black Friday. It fits – publicity, economic, symbolism. As an added bonus they would destroy Christmas. Just some twisted thoughts on the matter.