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