IMHO the Macy's Thanksgiving Day parade is the highest probability. It would accomplish every one of the goals cited for Al Qaeda. It does not need a nuclear device as conventional or improvised explosives could do the job. There is literally no way to secure that event. However there are a number of other very high profile targets in a number of areas. The Accela train is an easy target. and very high profile. However if they want to be truly spectacular then the Macy's parade is the big one.
Stay well - Stay safe - Stay armed - Yorktown