Add a mask, and you're all set. Then, prop them up around your yard, in whatever poses and actions you think will best fit with your theme yard. You might even want to use skeletons or dummies to be temporary drivers for your vehicles. Prop one up in your car, truck, open trunk, or on top of a tractor, the list is endless. Just take a look around your yard.
Ghosts are really easy to make, as well.
Find something of substance and cover with the sheet. Then, place the object in the center and draw the edges and corners of your fabric around, allowing the excess fabric to hang down for the flowing effect. Once you have a good selection to work with, simply hang them anywhere you can find space. Use tree branches, edges of the house, antenna of a car, off the mailbox. Just use your imagination.