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