Buy one or two rolls of white or cream colored medical gauze, and wrap your entire body with it. Use a safety pin to secure the material to normal clothes underneath the wrappings. Be sure to leave room for your eyes and mouth if you want to properly socialize at the event. Don't forget to drag a foot and to groan like the undead to act the part of an undead monster.
Put on an outdated and oversized flannel shirt and pair of jeans from a thrift store. Grab some worn garden gloves and insert some straw or hay into every pocket, opening and seam you can find in the clothes. Top the entire ensemble with a large gardener's hat, and you are now a convincing scarecrow. Try walking as if you are a boneless straw-filled creature with jerky emphasized motions.
To quickly become a terrifying werewolf monster, simply grab a cheap matching set of a mask and gloves from a costume store. All you need to do to complete the illusion of becoming a werewolf is to put on a long-sleeved shirt, pants and shoes. These sets are inexpensive, and they leave the user to complete the costume with normal street clothes. Alternatives to becoming a wolf-person include swamp-beasts and ape-people.