Instant bloody eyeballs. If you stir baking soda into vinegar, it'll foam right out of the glass. You can also shake salt over a lighted jack o lantern candle and make blue sparks. It is easier and a lot safer. They're just as startling, and they last longer.
Lighting is very important.
Portable spotlights, with blue lens covers, shed a ghostly cast. Swapping standard outside house bulbs with yellow, orange, or green bulbs achieves an eerie effect. For an exotic glow, station black lights where trick or treaters come knocking. Merely turning lights upside down can be effective also. Sound effects add a spine-tingling edge to spooky surroundings.