Here is the full list of all 37 Psychonauts achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Find all Golden Helmets.
Buy an item from the Camp Store on Christmas.
Complete Meat Circus.
Achieve Rank 101.
Sort all Emotional Baggage.
Gather all Figments.
Crack all Vaults.
Complete The Asylum.
Revisit all brains after completion.
See yourself through the eyes of many others.
Introduce all Camp Kids to Mr. Pokeylope.
Re-brain the Children.
Complete Waterloo World.
Complete Black Velvetopia.
Earn all Merit Badges.
Find Edgar's Secret Garden.
Complete Gloria’s Theater.
Achieve Rank 80.
Redeem all 16 Scavenger Hunt Items.
Complete The Milkman Conspiracy.
Complete Lungfishopolis.
Achieve Rank 60.
Spy on Bobby's love life.
Hear Vernon's Ghost Story.
Complete Milla’s Dance Party.
Uncover Milla's Secret.
Use the bacon. A lot.
Achieve Rank 40.
Complete Sasha’s Shooting Gallery.
Witness Maloof's transformation.
Cook and consume two different kinds of roast in one sitting.
Achieve Rank 20.
Give the Coach's speech on the stump.
Read many bulletin board messages.
Discover the secret history of Whispering Rock.
Complete Basic Braining.
Complete the Punchy Target mini-game.