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