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