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