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