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