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