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