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