Luigi's Mansion: Dark Moon
Capturing Boos are introduced as early as Gloomy Manor's "Visual Tricks" mission. However, the game does not explicitly tell you what is the reward for capturing them all.
E. Gadd Medals
You can obtain golden medals with E. Gadd's face on them that appear on your save file if specific requirements are met. There are three to collect.
#1: Complete the Dark Moon Quest mode.
#2: Obtain a 3 star rank on every mission in the Dark Moon Quest mode.
#3: Collect every type of ghost in game. (Both Evershade Valley and Scarescraper Ghosts)
Endless Floors in the Scarescraper
This option creates a Scarescraper with infinite floors as well as its own difficulty, also named "Endless".
Beat the mission "Visual Tricks" in Gloomy Manor.
Collecting all of a mansion's gems will unlock a statue that can be seen in the vault's gem collections.