save and/or find cardboard boxes and pieces of cardboard

tombstones

cut up pieces of cardboard boxes or cardboard pieces you have into a tombstone shape, paint or color or tape grey paper onto it, draw a black line and write RIP at the top and any other message you want
if you fold the cardboard in half and then cut the rounded shape it can stand alone like a place card

tablecloths

while I don’t condone using plastic, you can use ripped up trash bags as tablecloths or other spooky decor
go to your local thrift shop to find old lace and other fabrics
also putting white sheets on furniture gives a spooky vibe

candles

have a seance vibe with many different sizes and shapes of candles either that you have, or from the thrift shop, or DIY ones
Find some white candles, and place them upright, then take some red candles and light them and drip the wax from the lit red candle over the tips of white candles and let the red wax trickle down them to look like blood

ghosts

Find some white or light-colored fabric scraps (like old t-shirts or sheets) and cut them into squares of different sizes. For each ghost, bundle a small amount of stuffing (you can use crumpled newspaper, more fabric scraps, or organic cotton) in the center of a fabric square. Tie below the bundle with a string or twine to form the ghost's head. You can draw the ghost's face with non-toxic markers or sew on buttons for eyes. Hang these ghosts around your home or garden. They can be stored and reused annually, and if they ever wear out, the fabric can be composted or recycled if made of natural fibers.

flowers

the one time of year dead flowers work! either dry flowers you have or find stores that will offer supper discounts on nearly dead flowers

witches brooms

collect fallen twigs and bundle them together with string and you can make all different sizes

spider web

ripped tights
old sheets or net curtains and cut the lace curtain to make it look like a big cobweb and hang it
compostable floss (white or charcoal colored)

spiders and bats

pipe cleaners are not biodegradable but if you already have some or can buy secondhand, they’re perfect for making creatures
otherwise use black paper or draw on recycled paper

alien slime

Mix 1/2 cup of cornflour with 18-20 teaspoons of water and add food coloring or natural color for a gooey alien slime

drinks

use any jars and vases you find in your home, or thrift shops, and fill with food coloring, tea, juice, charcoal tabs, chlorophyll tabs, or colored alcohol (like purple gin), to make it feel like a mad scientist’s lab

eyeballs

peel radishes, place stuffed olives in them, and freeze them in an ice tray
freeze ice cubes with lychees and blueberries so they look like eyeballs
buy gummy candy eyeballs and freeze them, could even do a red juice as well to make them look bloody

bugs

Halloween