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