This is something a writing teacher I had years ago had my class do. I ended up revising mine into a short story, it was so successful. Anyway...
Your character is standing in a dimly lit hallway. It is empty save for the closed door in front of him/her. There is nothing remarkable about the door, but the thought of the scene beyond it fills your character with nervous tension. Your character takes a deep breath, reaches out to grasp the doorknob, and opens the door. Your character remains standing in the doorway for a moment, surveying a scene in the room.
Your task: Describe the room you character sees through the open doorway. The point is simply to describe it, not to describe your character or to have your character interact within the scene. Look at it through your character's eyes. Try to use description to evoke a feeling, and maybe explain why your character was nervous about entering. There can be other characters within the room. Do describe them if there are, but they do not interact with your character.
See if you can create something that will not only allow a reader to visualize the room, but will have them feeling the same tension your character felt.
See this thread for other writing challenges.