Here is a list of all the secret codes that you can activate in the game.
markiplier - Pink mustache
jacksepticeye - Sam's Septic Eye
lordminion777 - Crown
muyskerm - Headphones
jimsterling - Toy Boglin
POISONAPPLE - Apple
B-0V1N3 - Cow mask
00204-389-1902-5530 - Beta Octopus Hat
LORDGRUMGRUM - Shark cat