Minecraft wiki entry for Legend_of_the_Chambered, associated text: Some Niches.
Pentagonal Stone Slab (referred to as a pressure plate in the game files, but has no function in-game) with a Stair Well behind it. The player could not actually use them though.
The doors that lead into the woods.
Pit in the Woods. Players cannot go down it due to an invisible barrier.
The word "Win" on one of the walls. There isn't actually an end to the game.