From the Super Mario Wiki
At first, the left restroom is blocked by a pile of food inside, so Luigi can only enter the right restroom. There is a painting near the door to the right restroom Luigi can use the Dark-Light Device on to get coins.
Inside the right restroom, there are three stalls, two sinks, and a dryer. Luigi can use the Suction Shot on the right stall to pull off the door to discover a floating newspaper. He can then pull away the newspaper to reveal a Gem Goob with the white Gem.
In the left wall is a drain which Gooigi can go through to get to the left restroom. He is then able to clear away the pile of food blocking off the door, allowing Luigi to enter the restroom. There is also food in the sinks and a bag full of money hidden behind the sinks. There is an invisible dryer in the southwest corner of this restroom which can only be revealed with the Dark-Light Device. Gooigi can then use the dryer to get a lot of coins.
After the Mouse with the fourth floor elevator button flees the Entertainment Room, he runs to the left restroom. Luigi needs to use the Suction Shot to pull off the door to the middle stall to reveal a group of rats sitting on the toilet. He then needs to find the glowing one with the elevator button to get it back.