Making your own graveyard, or just highlighting your yard with just one spooky grave, can be a DIYers dream. Heavy duty cardboard, scraps of wood or even thick Styrofoam can be repurposed to make gravestones.
Cut your graves into various shapes and don't hesitate to use pictures from the Web for inspiration. Paint them gray, then write epitaphs for the dead in black paint or with a large marker. And don't forget to include a "freshly dug" grave in front (read: pile of dirt about the length of a human body).
Add to the fun (or is it fright?) by scattering body parts near the graves, maybe half-buried. Arms and legs are easy to craft - cut the arms and legs off of an old shirt and pair of pants, stuff with old rags or batting from the fabric store, and sew shut.