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