I hope you are all planning to see Harry Potter today (if you have not already). I made Oscar some felt Harry Potter glasses in celebration ... and if you cannot tell from the photo, they are wildly unpopular.
You can find lots of fun Harry Potter crafts here (I especially like the wands).
I'm not the type to dress in costume to go see a movie, but I did go to a Deathly Hallows release party as Hagrid a few years ago. All my friends were dressed up too, and people barely batted an eye at our group down in the subway. And that's why I loved NYC.