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