Some handy tips to keep in mind:
Have the proper attitude. You'll need to not be afraid to get really silly, to be a big enough fan that you enjoy messing around with all the minutiae, and to know when enough is enough and you're getting obsessive and it's time to stop.
Read the books. Especially if you've only seen the movies. You might even want to read a bit of it while everyone is arriving, or needs a little downtime. The books are so rich in detail and humor, and when you read them together as a family, it really adds to the fun.
Plan, plan, plan. Our day involved the guests getting sent an acceptance letter to Hogwarts on the school letterhead, and when they arrived, they were handed a notebook with the day's schedule and some activities to do until everyone was present. We made many, many, many things in advance: workbooks (Google "Harry Potter coloring pages" or "Harry Potter activities" to find loads of free resources), capes made out of trash bags for the kids to decorate and wear, pretend Potions supplies that we gleefully labeled - (like "powdered lacewings" that was really the contents of a jello packet), their wands, and the homemade packages of Bertie Botts Every Flavor Beans that Sophie chose the flavors for from this list. Fonts were from The Daily Prophet (Harry P and Lumos) and were printed on big Avery labels and cut out with fun scissors.
Have a schedule. The students were greeted by Dumbledore and Professor McGonagall, got sorted into their houses with the sorting hat, chose their wands, decorated their capes, and went to Potions class (messy things in the kitchen), Transfiguration class (Shrinky Dinks; we pointed our wands and yelled "Reducto!" at the toaster oven while they were cooking), and Divination class (we pretended to read their palms and predict funny things). As they completed each class, they got a sticker to put next to the subject in their workbook. At the end we had a graduation celebration with Butterbeer (root beer) and cauldron cakes (cupcakes). All of this took about an hour and a half but could have been longer if we'd wanted. The awesome thing is that we're doing it again this weekend for Peter's birthday!
HAVE FUN! I suspect a few of the parents thought we were nuts. We didn't care.