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