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