If you weigh fewer than 100 pounds, have I got the Halloween costume idea/business opportunity for you.
A recent posting to Ann Arbor's Craigslist advertised for a small person to hide in the poster's backpacker-costume backpack, occasionally bursting out to startle unsuspecting fellow party-goers. If you are claustrophobic or worried about muscle atropy, worry not: the poster is a "reasonable person" and will not make you stay in the pack for the entire night.
I wonder if I can lose 60 pounds by Halloween.
UPDATE: The Craigslist posting has been taken down since the time of writing.