Need some easy signs, monsters, or walls?
Then grab yourself some cardboard!!
WALLS
The best kind of cardboard for this is the thick kind like from big boxes, you can get flat sections that are usually around 4 foot by 4 foot at grocery stores sometimes. (from packaging) Line them up together and use silver duct tape to make a large sheet. Then tape together in a /\// -zigzag shape so it will stand up by itself. Then either drape black plastic over it or use spray paint on it. Using black lights? Use florsecent paint! Write words, or spray it for a few seconds in one spot and let it drip down!
MONSTERS
You can use a flat sheet as talked about above or use a piece from a box! Depending on how large you want your monster. Then with a pencil or pen draw a shape of a monster(head, arms, etc.) Then cut out! Use poster paints or spray paint to color your monster then spray a clear coat of protectant (you can find this at a arts and craft store) to weatherproof it. Nail or attach to a fence so it looks like your monster it looking at you over the fence or peeking over the fence, or have a arm come out through the fence! Another idea is put your monster peeking from behind a tree or garbage can.