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