Since Hogwarts is the school of witchcraft and wizardry, I suggest you have a theme party and have everyone dress as a wizard or their favorite character in the Harry Potter movie. Of course, if this is a birthday party, the birthday celebrant can be Harry Potter or Hermione and everyone else can choose other characters to dress up as.
I would have the Harry Potter dvd series ready and have it playing continually on a tv. Make sure someone is around to switch up the dvd's when one is over.
Use Harry Potter invites, eVites or save the dates to keep with the theme. Get a Harry Potter cake. The Wilton company has Harry Potter molds if you wish to make one yourself or you can special order one at your local grocery store or bakery.
Play games related to the theme of wizardy, such as best magic drink competition, but make it a drink recipe, or fastest flyer competition and make it a race where people hold broomsticks. There is also a Harry Potter game you can purchase to play as well.
You can also host a scavenger hunt and make sure all the tokens found are related to the Hogwarts School of Witchcraft and Wizardry. It could be the wizards scavenger hunt.