There are a ton of fairytale characters in each Shrek movie. Some of the costumes are available year-round in the Disney store (Snow White, Cinderella and Sleeping Beauty) and some you can make yourself using this guide as a reference.
Shrek: Use a long white shirt tied with a brown belt, a short brown vest, brown tights and brown shoes. Add to the look with green face makeup and Shrek ears.
Donkey: Wear a gray fleece shirt and matching sweat pants. Create gray and pink donkey ears by taping ear shapes to a headband. Then, make a gray donkey tail using felt and fake fur.
Princess Fiona (human): Wear a long emerald green princess dress, a red wig with braided hair and a tiara.
Princess Fiona (ogre): Wear a long emerald green princess dress, a red wig with braided hair and a tiara. Add to the look with green face makeup and Shrek ears.
Puss in Boots: Dress as an orange cat with long black boots, a black belt and European black hat with a feather.
Pinocchio: Draw one vertical line on each side of the child's mouth down to the chin to look like a wooden mouth. Wear a white shirt with blue suspenders, blue shorts and yellow cone hat. Add to the look with wooden moccasins, short white socks and white gloves.
Three Little Pigs: Dress in all pink with a pig nose, ears and a curly pink tail.
Wolf in Granny's Outfit: Dress in a wolf's costumes (fake fur, wolf nose and ears) and then put an old moo-moo or nightgown. Finish off with a night cap over the wolf outfit.