Spooky music: Imaginations will run wild when given very little stimuli. If you did nothing but turn out all the lights in your house and put a portable speaker or iPod dock on the porch blaring spooky music, you still would make passing trick-or-treaters squeal with delight.
Brown bag luminarias: Flickering candlelight adds just the right bit of mystery and mood. Grab some brown lunch bags, fill the bottom quarter with sand and stick in tea lights. You can also use empty plastic gallon milk cartons. Simply cut off the bottoms, peel off any stickers, and decorate the carton's main side with a ghost-like face. Place a votive candle or battery-operated votive inside and you have a ghost-lined walkway.
Sprinkle tombstones on the lawn: Cut out tombstone shapes from cardboard boxes you already have. If you have gray, silver or black paint, apply a coat. If not, just decorate the boxes with markers and place them around your yard.
Hang ghost balloons: Have some basic latex balloons blown up with helium at your local party super store. When you get them home, drape them with white sheets and voila -- you have ghosts floating in your midst.