And, obviously, Dementors would be involved.
I'm thinking something along the lines of the Shrieking Shack or the Chamber of Secrets, a scare house that differs from the norm in that it offers eeriness and mystery rather than gore and cheap scares. Additionally, it would be completely interactive, involving various puzzles, similar to the ones encountered in the books.
Say, for example, a room with several doors. However, all but one are fake projections. As you approach each projection, it fades into nothingness, leaving only one door left: the one you came in through. However, the hallways adjacent to the room shifted position while you were in the room and the door you came through now leads somewhere else.
There could be a painting/door that only opens when you answer its riddle, just like the Fat Lady. There could be a hallway with a door at the end that appears normal upon first glance, but gradually gets smaller as you approach, like an Ames Room:
They could get wicked creative. I'm tired and can't think of much at the moment, but give their creative team a pot of coffee and a lot of scrap paper and they'll come up with something.
Another good visual comes from the Quitich World Cup game. The American Quittich stadium is very Halloween!