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