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