Here is the full list of all 37 Psychonauts achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Buy an item from the Camp Store on Christmas.
See yourself through the eyes of many others.
Redeem all 16 Scavenger Hunt Items.
Achieve Rank 60.
Complete Gloria’s Theater.
Use the bacon. A lot.
Discover the secret history of Whispering Rock.
Achieve Rank 40.
Complete the Punchy Target mini-game.
Hear Vernon's Ghost Story.
Uncover Milla's Secret.
Cook and consume two different kinds of roast in one sitting.
Witness Maloof's transformation.
Give the Coach's speech on the stump.
Complete The Milkman Conspiracy.
Read many bulletin board messages.
Complete Lungfishopolis.
Spy on Bobby's love life.
Complete Milla’s Dance Party.
Achieve Rank 20.
Complete Sasha’s Shooting Gallery.
Complete Basic Braining.
Complete Waterloo World.
Complete Black Velvetopia.
Complete The Asylum.
Complete Meat Circus.
Achieve Rank 80.
Achieve Rank 101.
Sort all Emotional Baggage.
Crack all Vaults.
Gather all Figments.
Earn all Merit Badges.
Find all Golden Helmets.
Re-brain the Children.
Find Edgar's Secret Garden.
Introduce all Camp Kids to Mr. Pokeylope.
Revisit all brains after completion.