My favorite that I have run is as follows (warning, its a high level room with deadly traps...so pare it down if you are lower level)
Room is a 25x25 space with a 50'x10' hallway on the other side of the entrance. In the 25x25 room is a pit 20' wide and seemingly bottomless. a narrow path skirts around each side (2.5'). laid across the pit is a rickety looking, narrow (2' wide) and sagging wooden plank bridge with missing planks, loose connections, and rotten wood. the bridge points at the hallway beyond.
The hallway slopes up from the room and ends with a large double door. the hallway is heavily trapped with poison darts, wall/floor spikes, etc. These should be fairly easy to detect and not very damaging. Should the players skirt the edge of the pit and make it through the hallway to the doors at the end, the door will open to a 10x10 room with no other exit and containing a very large round stone sphere. Opening the door triggers a trap behind the door; a rolling sphere trap from the DMG, which will roll down the hallway, onto the rickety bridge, and disappear halfway across (its passage over the wood bridge and subsequent disappearance will be obvious to any conscious players)
The room is secretly a demiplane that the players entered when they stepped inside the room, and they will be unable to exit via the way they came in (one way portal). The exit is an invisible portal, located in the middle of the rickety bridge (which is actually sturdy, just cleverly disguised). Players can observe discrepancies with the wooden bridge with a DC 20 Perception check (visual inspection), a DC 15 Investigation check (physical inspection), or by stepping more than five feet onto the bridge (it gives if they just put their foot in it to test it). Success will indicate that the bridge is not actually wood, but cleverly disguised metal, and much sturdier than appearances show. the pit can be bottomless, or (at DM discretion if they don't want to autokill their players who fall) is 60 feet deep, and the bottom 20 feet are under permanent effects of darkness and silence spells
The presence of magic at the center of the wooden bridge can be detected using Detect Magic (conjuration magic) but physical interaction will be needed to determine that the magic is a portal (it is also one way). The hallway traps can be detected with DC 15 Perception traps, and the rolling sphere trap can be detected via a DC 20 Perception trap (which will only uncover that something (the sphere) is pushing on the other side of the door).
If the players discover the portal and figure out its use, passing through it will bring them out of the demiplane into the material plane, into a room the exact same size and shape (minus the pit). The real hallway is flat, untrapped, and the door at the end opens easily allowing the party to advance to the next space.
the players are in a tall room, 30x60. They will immediately notice that their footsteps and voices echo greatly in the room. At the corners of the room are four golden colored statues and a door is located at the far end, upon which are written the words "silence is golden" The door cannot be opened by physical or normal magical means.
three of the statues are fools gold (pyrite) which can be detected via an appropriate Perception check. The fourth is real gold. Behind each statue is a small lever. The gold statue lever, when activated will cast the silence spell on the room for 1 minute. While the spell is active, the door can be opened. Alternately, the party can cast the silence spell on the correct statue to open the door while the spell is in effect. The pyrite statues levers do 1) nothing, 2) a massive cacaphony of sound that forces all creatures in the room to make on a DC 15 CON save, taking 4d8 Thunder damage on a fail, and half on a success, and 3) casts the blindness/deafness spell on the creatures in the room (Save DC 15), deafening them on a failure.
My favorite that I have run is as follows (warning, its a high level room with deadly traps...so pare it down if you are lower level)
Room is a 25x25 space with a 50'x10' hallway on the other side of the entrance. In the 25x25 room is a pit 20' wide and seemingly bottomless. a narrow path skirts around each side (2.5'). laid across the pit is a rickety looking, narrow (2' wide) and sagging wooden plank bridge with missing planks, loose connections, and rotten wood. the bridge points at the hallway beyond.
The hallway slopes up from the room and ends with a large double door. the hallway is heavily trapped with poison darts, wall/floor spikes, etc. These should be fairly easy to detect and not very damaging. Should the players skirt the edge of the pit and make it through the hallway to the doors at the end, the door will open to a 10x10 room with no other exit and containing a very large round stone sphere. Opening the door triggers a trap behind the door; a rolling sphere trap from the DMG, which will roll down the hallway, onto the rickety bridge, and disappear halfway across (its passage over the wood bridge and subsequent disappearance will be obvious to any conscious players)
The room is secretly a demiplane that the players entered when they stepped inside the room, and they will be unable to exit via the way they came in (one way portal). The exit is an invisible portal, located in the middle of the rickety bridge (which is actually sturdy, just cleverly disguised). Players can observe discrepancies with the wooden bridge with a DC 20 Perception check (visual inspection), a DC 15 Investigation check (physical inspection), or by stepping more than five feet onto the bridge (it gives if they just put their foot in it to test it). Success will indicate that the bridge is not actually wood, but cleverly disguised metal, and much sturdier than appearances show. the pit can be bottomless, or (at DM discretion if they don't want to autokill their players who fall) is 60 feet deep, and the bottom 20 feet are under permanent effects of darkness and silence spells
The presence of magic at the center of the wooden bridge can be detected using Detect Magic (conjuration magic) but physical interaction will be needed to determine that the magic is a portal (it is also one way). The hallway traps can be detected with DC 15 Perception traps, and the rolling sphere trap can be detected via a DC 20 Perception trap (which will only uncover that something (the sphere) is pushing on the other side of the door).
If the players discover the portal and figure out its use, passing through it will bring them out of the demiplane into the material plane, into a room the exact same size and shape (minus the pit). The real hallway is flat, untrapped, and the door at the end opens easily allowing the party to advance to the next space.
Another one that is not (very) dangerous:
the players are in a tall room, 30x60. They will immediately notice that their footsteps and voices echo greatly in the room. At the corners of the room are four golden colored statues and a door is located at the far end, upon which are written the words "silence is golden" The door cannot be opened by physical or normal magical means.
three of the statues are fools gold (pyrite) which can be detected via an appropriate Perception check. The fourth is real gold. Behind each statue is a small lever. The gold statue lever, when activated will cast the silence spell on the room for 1 minute. While the spell is active, the door can be opened. Alternately, the party can cast the silence spell on the correct statue to open the door while the spell is in effect. The pyrite statues levers do 1) nothing, 2) a massive cacaphony of sound that forces all creatures in the room to make on a DC 15 CON save, taking 4d8 Thunder damage on a fail, and half on a success, and 3) casts the blindness/deafness spell on the creatures in the room (Save DC 15), deafening them on a failure.