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