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