One thing that I've always really loved about Horror Nights is their attention to detail when it comes to their ideas for certain beverages and merchandise. You'll find most of your horror night "unique" merchandise on the street. Definitely pay attention to the way they serve alcohol this year. Look out for an item by the name of the "Usher Gusher".
As for Merchandising, the event T-shirt seems to be the biggest seller. Since it's your first time going I'd definitely stick to something like that to commemorate your experience. :)