Cosmetic Effects
There are NPCs and items that Waka can interact with to change her sprite, but they don't seem to have any other practical uses.
Racket
When Waka first enters the dream world, interact with the red racket above the music box. This can also be toggled via the dark grey NPC in the monochrome twins room.
Monochrome
Enter the west door from the nexus. Head north until you see a wall of metal bars and then go east and enter a house in the middle of the room. Interact with the light grey twin. Waka can also hold her racket while monochrome.
Neon
Go through the north door in the nexus. Go to the purple star pattern on the floor and head east until you see a yellow triangle NPC. Go a little south and look for a fast-moving NPC with a yellow glowing head. Interact with it to change Waka's sprite.
Items
Neon-kun
In the nexus, enter the north door. In the new area, head south until you see an NPC with a pink head and green body. Interact with it to get the item. However, using the item when you have less than full health with replenish Waka's health, but freeze the game, forcing you to close and reload the game.
Endings
Ending 1
Enter the west door from the nexus. Go south-west and interact with the woman in front of the desk, choose the second answer to trigger End 1. Triggering this ending increases Waka's level by one, but it doesn't seem to have any other effect in the game.
Ending 2
Interact with the same NPC as ending 1, but choose the first answer instead. After the cutscene, enter the doorway in the new area. Head north up the hallway and leave.
Ending 3
From the nexus, enter the west door. Follow the train tracks south. In the maze, go straight south until you reach the end of the path. Go right and interact with the blue triangle NPC. In the new area interact with the NPC on the right. Talk to the girl in the house.