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