A snuggie.
ETA: Ok serious now, I would suggest a blanket (although i'm an avid snuggie-hater) but if you anticipate standing and sitting a lot, then a blanket would be useless. I would find a really warm jacket/pullover or the like. My dad has this reeeeeeeally warm Giants pullover that I used when I went up in the mountains to go play in the snow. It kept me not only dry, but super warm! I coupled that with some head covering (I wore a baseball cap, and then pulled a knit cap over it) and some gloves, and I was set. That and some "long johns" under some pants should keep you nice and warm.