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