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