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