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