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