A medium sized room with a flagstone floor. It could potentially contain any variety of set dressings. It could be an audience chamber, or a library, or whatever else is appropriate for the dungeon. However, the task may confuse players if the path between the two secret doors is blocked by anything large.
Two of the flagstones in this room are pressure plates. Each player walking through the room has a 1-in-20 chance of stepping on one of them. Any players who perform a task in the room which would require more movement than simply passing through, have a 1-in-6 chance to activate one of the flagstones. If a flagstone is stepped on, a secret door opens in the wall roughly 15ft away. This door remains open only so long as the flagstone is held down, and will close if the person standing on it steps off.
Inside of each secret room is a monster. The type of monster is not terribly important, though a construct or undead may be the most thematically appropriate. As I imagine it, the same type of monster exists in both rooms. However, if you wished to play to the room’s theme, you might put a brute of a monster in the “body” room, and a cunning or spellcasting monster in the “mind” room.
Regardless of what monster is used, a new one appears in the secret room each time the door is closed. So if a player steps on the pressure plate, releases the monster, and steps off the pressure plate to fight it; then when the player returns to the pressure plate to open the door again, another monster identical to the first will be released. The pressure plate can be held down by any small object weighing at least 50lb (it must have a base small enough that its weight isn’t distributed to other flagstones). An iron spike would also suffice.
Both secret rooms are small, only 10x10ft, and aside from their monsters, they have only one distinguishing feature. On the back wall of the rightmost room is a strawberry-sized, ceramic human brain, painted pink and grey. On the back wall of the leftmost room is a ceramic human man with a muscular frame, and a large cavity in the back of his head. Players who have already discovered and examined the “brain room” will notice that the cavity on the back of the figure’s head is just about the same size as the brain. (Though, of course, both rooms must be discovered interdependently of one another. So players who find this little man may not know a properly sized brain is nearby).
If either ceramic piece is pulled upon by anyone, it easily comes free of the wall, and the puller will discover that it is attached to a strong, steel cord. If the object is released, the ceramic cord will reel back into the wall, pulling the object back into its place.
The cord unreels easily enough, until the object reaches the doorway of the secret room. Once here, players will find themselves completely unable to pull the cord another inch unless they meet the appropriate ability score prerequisite. Only players with a Constitution of 12 or greater can pull the figure of the man’s body beyond the doorway. Likewise, the brain can only be removed from its room by players with an Intelligence score of 12 or greater. If need be, a player who does not meet these ability score prerequisites can hold one of the objects to prevent it from reeling back into its room, but it will require all of their strength to do so.
If the party manages to pull the two pieces together, and place the brain inside of the body, then the cords will disappear, and the tiny man will drop to the floor and come alive. From seemingly nowhere, he will pull out a grappling hook, and throw it up through a hole in the ceiling (which before that moment was solid stone). He will then scramble up the rope, and out of sight, leaving the party alone with the rope leading up into a secret passage.
OPTIONAL: The tiny man steals something of value from whoever was nearest to it at the moment it came alive, and flees with that object through the passage. When the party encounters him again, they will discover that despite his tiny size he is incredibly strong, and brilliantly intelligent.
Not every party will be able to solve this room. Regard that as a feature, or as a bug, as you will.
Note that one of this room’s benefits is that it (eventually) produces an positive result from the normally deadly blunder of stepping on a pressure plate. In future, players will be forced to wonder whether the pressure plates they encounter ought to be avoided entirely, or experimented with.