Get a white sheet and drape it over yourself and call yourself a ghost.
Reuse the same sheet for next year and splash fake blood onto it and call yourself a bloody ghost.
Wear all black and attach two black triangles cut from construction paper onto a black headband.
Find the trashiest clothes that you own and wear them, you shouldn’t have to explain it.
Make a sword out of cardboard and find an eye patch to become a pirate. Bonus points for thrifting a plush parrot.
Find an outfit that just has random splotches of colour everywhere. You’re suddenly an abstract painting.
Get a large amount of those ‘Hello, My Name is’ labels, fill them out all out with random names and plaster your clothes with them and you’re suddenly Confused or an Identity Crisis.
Outfit yourself with a box with arm cutouts and a box for a hat and you can go as a robot.
Get the same white sheet as the first suggestion and fashion a toga out of it.
Find an incredibly long wig and attach a bunch of leaves to it and goes as Autumn to that costume party.
Wear your comfiest pjs, draw a few freckles onto each cheek, get a pacifier and carry around a teddy bear.
Find a disgustingly bright orange jumpsuit and pin a series of numbers on the back and you can be an inmate.
Wear whatever you normally wear and claim to be something extravagent… in disguise.