Here's several that I've seen done (or done myself) in years past:
Your desk: if you work in an office, take a large cardboard box, attach straps to go over your shoulders and around the waist, and wear the box, bottom-up, at waist height. Attach the usual items from your desk: cluttered papers, a laptop, box of tissue, desk phone, stapler, etc.
a tree: wear brown pants and a green top. Cut small branches from a tree and lash them to your arms, leaves and all. Wear a cap and pin leaves to that as well. More branches/leaves make a better costume, but become unwieldy and somewhat messy. Further ideas: If you have fruit available, lash a piece of fruit to either end of a length of string and wear that around your neck or to other body parts (waist, elbows, etc). For the deluxe version, add details like pinecones, magnolia blossoms, a plush squirrel or owl, lianas, mistletoe, etc.
Dress like your favorite character: Here I don't mean the obvious superheroes or fantasy film characters like Harry Potter. I mean contemporary normal people characters like Marty from Back to the Future, Lloyd from Say Anything, or Ferris Bueller. The costumes are easy to create, generally, because they use normal clothes, but they are distinctive clothes nonetheless.
If you are at school still, switch places with a friend: exchange your wardrobe, mannerisms and dialect for the day (assuming that your clothes fit each other or that you can reproduce their favorite clothes pretty well with your own).
Your desk: if you work in an office, take a large cardboard box, attach straps to go over your shoulders and around the waist, and wear the box, bottom-up, at waist height. Attach the usual items from your desk: cluttered papers, a laptop, box of tissue, desk phone, stapler, etc.
a tree: wear brown pants and a green top. Cut small branches from a tree and lash them to your arms, leaves and all. Wear a cap and pin leaves to that as well. More branches/leaves make a better costume, but become unwieldy and somewhat messy. Further ideas: If you have fruit available, lash a piece of fruit to either end of a length of string and wear that around your neck or to other body parts (waist, elbows, etc). For the deluxe version, add details like pinecones, magnolia blossoms, a plush squirrel or owl, lianas, mistletoe, etc.
Dress like your favorite character: Here I don't mean the obvious superheroes or fantasy film characters like Harry Potter. I mean contemporary normal people characters like Marty from Back to the Future, Lloyd from Say Anything, or Ferris Bueller. The costumes are easy to create, generally, because they use normal clothes, but they are distinctive clothes nonetheless.
If you are at school still, switch places with a friend: exchange your wardrobe, mannerisms and dialect for the day (assuming that your clothes fit each other or that you can reproduce their favorite clothes pretty well with your own).