It could be a ghost of a child who died in the playground long ago. The child was locked in that room by accident and perished. Now its spirit haunts the space behind the locked door, and if someone opens it, they'll be greeted with a cold presence and the sound of the child's sobs echoing in the small, dark space.
Maybe it's a horde of rats that have mutated into something monstrous. They've been living in the dark space behind the locked door, feasting on god - knows - what. If the door is opened, they might pour out and swarm over anyone nearby, their beady eyes and sharp teeth glinting in the dim light.
The scariest thing could be a deranged clown. Imagine a clown that was locked away long ago for scaring kids too much. It's been trapped behind that door, and now it's angry and waiting for someone to open the door so it can have its revenge on the children in the playground.
A ghost. Just the idea of an otherworldly apparition lurking behind the door, waiting to make itself known, is terrifying.
Once upon a time, in an old, dilapidated playground. There was a locked door at the back of the abandoned equipment shed. One night, a group of kids dared each other to go and touch the locked door. As soon as the bravest one approached it, he heard strange whispers coming from behind the door. His hands felt ice - cold when he touched the rusty doorknob. He ran away as fast as he could, and the others followed, all feeling a sense of dread.
Maybe it's a magical world or a place where the secrets of the forest are stored. It could also be a place that holds the key to the character's past or future.
For me, the scariest part of the 1999 story was the Columbine shooting. It was so unexpected and the violence was extreme. Innocent students and teachers were just going about their normal day and then suddenly faced this horror.
The scariest thing could be a ghost. You can't see it clearly in the dark, but you can feel its presence. It might pass by you with a cold breeze, or you might hear its eerie moans.
The scariest thing could be seeing unexplained apparitions. Just imagine seeing a figure that looks like a patient who passed away long ago suddenly appear in front of you in a dim hospital corridor. It gives you the chills.
The scariest thing could be the feeling of being watched when you're all alone in a strange hotel room. It gives you this sense of unease and makes you constantly look over your shoulder.
The scariest thing could be the loss of control. As the '69 countdown' proceeds, things start to go wrong. You can't stop it. You might be trapped in a room where the temperature is dropping rapidly, and strange symbols are appearing on the walls. And when it hits zero, you don't know what horror awaits, whether it's being dragged into a dark void or something even worse.
It could be a hidden treasure or a secret passage.
Once, I heard strange scratching noises coming from behind the door at night. I slowly opened it, and there was a dark figure that quickly disappeared. I couldn't be sure what it was, but it left me with a feeling of unease for days.