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