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