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