Luigi's Mansion - Watering the Plant (Gold Diamond)
Where Spooky is in Area 2 of Luigi's Mansion you'll also find a small patch of dirt and in that patch of dirt a seed. If you water the seed the first time you're here it'll sprout into a little plant. Remember this area and remember to come back here because the reward you get for doing this is huge!
After you defeat Bogmire return here and water this plant again to have it grow larger still. Then again even later in the game Boolossus you'll come back here and water the plant a third time and it'll burst rewarding you with a ton of treasure including one of only two Gold Diamonds in the entire game. The second Gold Diamond is from collecting all 50 Boos.
The above picture is after watering the seed your first time. The picture below these sentences is after watering the plant a second time (right after defeating Bogmire in the other room which you reach through inspecting the doghouse).
The screenshot below is before you water the plant the final time. After you do it bursts and money flies everywhere. Vacuum it all up and don't forget to grab the Gold Diamond too!