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