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