If you start now, you can age your garden for autumn and all the spookiness -
Add some dolls or scarecrows, let leaves fall where they may, add dead branches, headstones, creepy solar lights on stakes for night time gentle glow -
Consider making creepy Blair-Witch-esque stick figures from the materials in the yard -
sticks, clay, twine, and paint -
If you have any old trellises, take them apart, make crosses and stick them in a fake cemetery part of the yard.