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