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