So we're big into player agency when it comes to finding traps and secret doors. None of that roll-a-die-to-see-if-you-find-it nonsense for us! But what do you do when you've done the moose head, the candle sconce, and the bookshelf gimmicks to death? Here are a few secret door triggers that aren't quite as cliche.
Show me the weigh: A balance scale is affixed to the wall near the location of the door. If a certain amount of weight is placed in each basket, the door is activated. If desired, a note with a cryptic hint, such as "Which weighs more, a pint of ale or a dagger?" may be left nearby.
Under pressure: There's a pressure plate in the floor, either near the secret door or on the opposite side of the room from it. The weight of a character or equivalent triggers the door. More fun if the plate is far from the door and nothing heavy is close to hand to keep the door open for the last character. A variant might have two or more pressure plates, requiring all to be depressed simultaneously in order to open the door. As an added bonus, players might take this for a trap rather than a secret door mechanism, and give it a wide berth!
Taking a load off: As the pressure plate, above, but the door's default position is open; it is held
closed by the weight of an object on the plate. When the object is removed,
the door opens. Of course, it might be opening to release a guardian creature to prevent the theft of the object...
The toll door: Examining the wall reveals a tiny slot, about as wide and as thick as a coin. If a coin is inserted, the door opens. A carefully designed mechanism might be able to distinguish coin types by weight, and only accept gold or platinum.
Beer me: A relief of a face with open mouth is carved on or near the door. Pouring a pint of some sort of liquid down the mouth fills a reservoir, triggering a weight-sensitive mechanism that opens the door and empties the reservoir. If the door is allowed to swing closed, the face must be "filled" again.
Opens with keys: A piano or organ sits against the wall with the secret door. Playing a certain series of notes activates it. A piece of sheet music nearby may hold the proper sequence.
Down the drain: Diverting water down a particular drain drives a water wheel that opens the door. Ideal for baths, sewers, or anywhere else with either plumbing or flowing channels of water.
Don't stair: The secret passage is hidden behind a staircase, which is either lifted or cranked up.
A question of alignment: A doorway is clearly visible, but appears to be a blind portal, blocked by a wall behind. The entire wall containing the doorway is made to slide, but this will only be noticed by examining the corners of the room, in which case the seams will be seen. When the wall is slid in the proper direction, the doorway in it will align with the opening in the wall behind it.
The king's treasure: A variation on the pressure plate theme. The catch to the secret door to a treasure room is concealed behind a standard sliding panel. However, it will not function unless the throne is occupied.
The bullseye: The room on the other side can be seen through an arrow slit, but no door is evident. On the wall in the other chamber is a target. Hitting the bullseye with an arrow or quarrel (a roll against AC 2) triggers the secret door between the two rooms.
With a twist: In a room with columns or pillars, one has a neat hole bored all the way through. This is meant for a stout steel bar to be thrust through, forming a lever to rotate the pillar and activate the secret door. The bar may or may not be present in the room.
Blow it up: A strange round hole in the wall is the only visible sign. A length of small diameter metal pipe may be found nearby, which fits exactly into the hole. Doing so and blowing into the pipe inflates a leather bladder mounted in the space within the wall, which presses upon the latch mechanism of the door, opening it.
Sham slime: The door is actually a sheet of oiled cloth treated to look like green slime, grey ooze, or yellow mold. It's easy to see, but not obvious that it conceals a portal, and...Who wants to be the first to touch it? May be highly flammable, for a nasty surprise if someone tries to preemptively burn the monster.
Fire in the hole: Near a fireplace or forge is a hole which looks to be a perfect match for the poker or other metal implement. Merely inserting it cold is ineffective; the poker must be heated to red hot first. When this is done, the hot poker heats a strip of heat-sensitive metal, causing it to expand and release the catch of the door.
The Slim Jim: The stout catch of the secret door is on the other side. It can be lifted free by sliding a long flat object, like a sword blade, through the seam.