Cut out the shape of cats or bats in construction paper. Write important party info on these. Hand deliver to invited guests. Hang black crepe paper and cobwebs everywhere. Hang black silhouettes of cats and bats throughout the house.
Black balloons are a nice touch.
Buy inexpensive eye masks at party store. Write important party information on mask and hand deliver to guests. Make a coffin out of cardboard and leave at entrance way for kids jackets etc. to be placed inside. Hang black and green crepe paper or streamers. Write party info in the form of an epitaph.