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