A few weeks ago, when we were in New York City for Book Expo America, we went to see the Harry Potter exhibit at Discovery Times Square. This is a wonderful exhibit of props, costumes, and set pieces from all of the movies, and I highly recommend it to any Harry Potter fans. Even if you aren't a fan of the movies (I've never liked them as much as the books) it's still a wonderful experience to see real-life versions of all the amazing things from the books: the Fat Lady, the Sword of Gryffindor, the horcruxes, even Dobby and Kreacher.
The exhibit is organized into thematic groupings, such as Quidditch, the common room, and the various classes such as Defense Against the Dark Arts and Potions. You can see the Marauders' Map and try to puzzle out the margin notes in the Half-Blood Prince's potions book. There are even a few interactive areas where you can sit in Hagrid's chair (it's big!), throw a Quaffle, and pull up mandrakes.
The exhibit runs through September 5 and advance tickets are recommended.