Backlight the webbing with candles so the effect shows through to the outdoors. Attach spiders to the webs as well. Dim the lights and light candles in strategic places.
Candelabras are a nice touch.
Use Christmas lights for more lighting while maintaining a feel of darkness. Make tombstones out of Styrofoam board. Paint them with silver paint and mark them with epitaphs. Then, secure them with stakes in the front yard. Use a sheet to drape over a large, round balloon.