Bonus Challenge (Luigi's Mansion: Dark Moon)
From the Super Mario Wiki
Bonus Challenges are small pocket levels that appear in Luigi's Mansion: Dark Moon. They can only be accessed via portals that often appear inside furniture or objects, and teleport Luigi into the Paranormal Dimension when he interacts with them. They usually leave him on small floating islands or in small mazes.
To access a Bonus Challenge, Luigi has to locate a shaking door. They can be found on fridges, coffins, or as fake doors, and are usually hidden from view or behind fake walls, requiring the player to hunt them down. Opening a shaking door pulls Luigi into a smaller level, where he must complete a set of tasks or minigames within a set time limit; these range from collecting several Red Coins and normal Coins in a maze or finishing a snowman's face, to capturing five Gold Greenies before they escape or having a snowball fight with some Hiders. The game ends when time runs out or Luigi completes the given objective, which rewards him with a Gemstone or a large amount of treasure, and returns him to the area he used to access the dimension.