Bring ghosts and witches to life in your yard. All it takes is some balloons and fabric. Ghosts are one of the easiest to conjure up - inflate balloons with helium and cover them with white sheets or white plastic trash bags. Use black marker to give them spooky faces and tie them to tree branches. Don't need your ghosts to float? Use a regular balloon or stuff the ghost's head with crumpled newspaper.
Make other monsters and zombies on a dime with old clothing - flannel shirts, overalls and T-shirts, for example - stuffed with leaves. Use a real or plastic pumpkin for the head instead of a balloon (unless you'd like your zombies to hover).
Use a similar formula for your outdoor witches. Simply use a black dress from your closet or hit the discount fabric store for some black remnant material. Accessorize your with a broomstick and pointy hat. To make the perfect accompanying prop, paint a large flowerpot black and add dry ice to create a witch's cauldron.