I like my secret doors to be puzzles -- finding the secret door doesn't give you any information about how to open it. I think this is interesting.
If your adventure is constructed so that the secret door is required to advance, as in you can't complete the scenario without discovering the secret door, then what's your goal? If the players miss the secret door (which is both possible and likely) then they are engaged in the tedious complex task of searching every room and fighting a ton of battles.
That's the trade-off, don't you know? You spend a turn, roll your 1 in 6 or 2 in 6 or whatever chance to find the door.
Normally I place the secret door, randomly determine it's mechanism, then I go back and add that/those items to the room, along with some Red Herrings nearby. They can search and discover the door; then try to figure out how to open it, or dick around with the mechanisms and be surprised when the door opens.
So let's assume you aren't into tedium, and you're bright enough to only include optional (but cool) content behind secret doors. Or, you're running a module.
Holy crap! 27 secret doors! That's Bullsh*t!
So, for your edification, here are some basic secret door mechanisms that you can apply to any old random secret door allowing you to hand back agency to your players.
- The secret door is a wall that swivels on its center point or on an edge.
- You have to pull a nearby sconce, idol, or book to open the secret door.
- There is a wardrobe or armoire with a open back, hidden behind clothes, or a false back that opens with a simple push.
- The secret door slides to the side.
- The secret door lifts up like a garage.
- The secret door opens just like a normal door, it just matches the wall exactly.
- A stone when pressed will open the secret door.
- A hole must have a long length of steel inserted to be able to move the door.
- The secret doors are actually just illusionary walls. They can be walked through with no difficulty.
- The secret door is actually just a block of stone that rests in the wall. It can be pulled out and set aside to enter the passage-way.
- Stairs lift up with a solid tug on the railing.
- A 'trap door' lifts up on the ground. The cover is shaped like a normal large stone. A cursory examination will revel the handle.