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