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