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