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