Spiders on the floor, spiders on the ceiling, spiders on the wall and on the lampshade…
Spiders scream "Halloween," and it's easy enough to make your own out of black cardboard, felt or fabric.
If you can draw, by all means go freehand; otherwise, pick up some spider stencils at a hobby shop or make your own using tracing paper and a picture of a spider. The Internet will have plenty -- try Country Living's Halloween Templates or FreePrintable's Spider Stencil. The only other things you need are scissors and a pencil, and you've got yourself some spiders.
Pair them with store-bought spider webbing or some torn-up gauze for a full-on spider theme. Hang them from a chandelier or a mantle, trail them up a banister, or stick them on a wall, a lampshade, the floor or the door.
If you laminate them, you could even stick them on the walkway to your door to give approaching trick-or-treaters a scare.
Or, you could go the supernatural route…