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