Land's End Canvas: Women's Heritage Pea Coat
A trench - or some variation thereof - gives a polished finish to every outfit, travels well (i.e. wrinkle resistant), and provides an extra layer of coverage for unseasonably cool weather at your final destination. I adore the "persimmon" version of this trench, but it also comes in more traditional shades of navy and khaki. Tech bonus: an inside pocket for your MP3 player of choice.