Frightfully Fun Fog
Dry ice is an old favorite for creating a creepy Halloween atmosphere with the billows of foggy mist it gives off. It's available at most places that sell fresh fish. Besides the old "smoking punchbowl" trick, you can also use dry ice to cover the entire food table in a layer of creeping mist, giving the impression that your guests are plucking their refreshments from the middle of a haunted swamp or graveyard.