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