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