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