Destruction Button (Trap Idea)
Here's one for you DM's that want a obvious trap, that is so obvious, that your players might fall for it!
The players have just fought a boss in a dungeons, & their appears to be a hidden room behind the boss room. If the players go into the room & search about, they fill find a button. This button read "destruction" button.
If the players, for some reason, hit the button, the room will start glowing read. A screaming alarm will play a waring that "The premises will blow up in [x] minutes". If the players don't get out in the allotted time, they will be subjected to the damage from the explosion (it deals more damage the farther away they are from the exit). After this trap, make most of the dungeon unacceptable, but leave the rooms that have any plot importance mostly intact.