You could spookify your toilet by not cleaning it for a month before Halloween, but I wouldn't recommend it. Instead, try Canadian Nickel Scrap'n's absolutely, hilariously delightful Spooked Out Toilet idea. Simple to replicate, simply use black scrap paper, tape, and some clever cutting. Head on over to Canadian Nickel Scrap'n for this and more penny-pinching Halloween decorating ideas.
Halloween Decor—Pennies a Project [Canadian Nickel Scrap'n]