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