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