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