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