Are you envisioning a haunted graveyard? It’s easy to get pre-made tombstones online ($15, partycity.com). But if you want to customize your headstones (by including the names of your neighbors or reflecting your theme), you can make them yourself with insulation boards ($10, homedepot.com). Lowes shows you how to do it here.
Are zombies more your thing? Use a dollar-store plastic skull mask as a mold and fill with spray foam insulation ($9, amazon.com). Once dry, push the foam from the mold and repeat until you have a flight of undead heads. You can paint them, but avoid spray paint, as the skulls will disintegrate. Scatter the heads across the yard, along the walkway or pin to the side of your house. You can find more detailed instructions here. (Note: Insulation foam is flammable, so keep it away from lit candles or jack-o-lanterns.) Complete your zombie apocalypse by stuffing old clothes with newspapers, creating the illusion of scattered dismembered body parts.
Thinking of a Victorian laboratory scene? Use that dingy, broken thrift shop (or dumpster) table as an antique haunted lab table. If that card table is not as scary as you’d like, coat it with rust or silver spray paint or just use a crackle-top finishing spray ($7, hobbylobby.com).