I wrapped cardboard boxes in plastic dollar store tablecloths. We collected a few dozen paint chips in various shades of green from the hardware store and glued them on for that Minecraft pixel look. I cut the eyes, nose and mouth shapes out and glued black pantyhose on the inside of the mask to cover the holes - opaque black from the outside just like Creeper, but see-through from the inside so my son could walk around safely at school all day and trick-or-treat through the neighbourhood at night.
My son was out of his mind with excitement when he saw the finished product. I'm pretty proud of that homemade Creeper costume!