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