The weekend before Halloween is always busy, and the city is filled with acitivities to keep you in the spooky spirit. Besides the Haunted Houses in different boroughs and Hell House at St. Ann's in Brooklyn, there are walking tours on Saturday and Sunday at the The Green-Wood Cemetery starting at 1PM. The Parks Department also has many different activities, including "Boo at the Zoo" events, Tompkins Scare Park, and the Pier of Fear.
Just check out raindates, as Saturday will be WET. Already, Prospect Park has cancelled the Haunted Walk on Saturday (there are more events to check out, though, for Sunday) and the Great Pupkin event at Fort Greene Park has been moved to Sunday.
New Yorkology has an excellent list of Halloween Happenings, like ghost storytellers at the Merchant House Museum and trick-or-treating at the Museum of Natural History.
Photograph of Green-Wood Cemetery by Benny Snyder/AP