Food: Harry's favorite dessert is treacle tart, a classic English dish made of a pie crust filled with golden syrup. If you can't find one at a local bakery and don't want to make your own, try pumpkin pie. It's a seasonal treat that looks similar to treacle tart.
No party is complete without drinks. In Hogsmeade, Harry, Ron and Hermione feast on Butterbeer. J.K. Rowling has said that she imagines the drink to taste a bit like butterscotch. Create your own by making milkshakes with butterscotch syrup or an ice cream float with cream soda.
Décor: Is there a better place to watch Harry Potter films than in Hogwarts? Decorate your home to look the School of Witchcraft and Wizardry by creating banners for the four houses—Gryffindor, Ravenclaw, Hufflepuff and Slytherin. A classic witch's hat can double as the Sorting Hat, and setting out broomsticks can make your living room feel like the Quidditch pitch.
Costume: It's boring to watch movies in your pj's when you can watch them in your wizarding garb. Slap on a black robe, a maroon scarf and a pair of round glasses, and you can be part of the Hogwarts gang. For the Harry wannabes, use face paint or eyeliner to create a lightning scar. If you'd rather be Hagrid or Dumbledore, a long fake beard will do the trick.
Goody bags: A classic Harry Potter treat is Bernie Bott's Every Flavor Beans. In the wizarding world, these jelly beans come in every flavor—from avocado to ice cream to vomit—but who wants to serve their guests candy that tastes like throw up? Send your guest home (or to bed) with bags full of unusually flavored (but delicious) jelly beans.
Other parting gifts could be individual wands or personal invisibility cloaks. The best part of a Harry Potter party is that the world is entirely fantasy—so let your imagination run wild!