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.
Crack all Vaults.
Complete Basic Braining.
Complete Sasha’s Shooting Gallery.
Complete Milla’s Dance Party.
Complete Lungfishopolis.
Complete The Milkman Conspiracy.
Complete Gloria’s Theater.
Complete Waterloo World.
Complete Black Velvetopia.
Complete The Asylum.
Achieve Rank 20.
Achieve Rank 40.
Achieve Rank 60.
Achieve Rank 80.
Earn all Merit Badges.
Redeem all 16 Scavenger Hunt Items.
Find Edgar's Secret Garden.
Uncover Milla's Secret.
Use the bacon. A lot.
Witness Maloof's transformation.
Achieve Rank 101.
Sort all Emotional Baggage.
Gather all Figments.
Find all Golden Helmets.
Re-brain the Children.
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.
Read many bulletin board messages.
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.