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