Here is the full list of all 37 Psychonauts achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Introduce all Camp Kids to Mr. Pokeylope.
Re-brain the Children.
Find Edgar's Secret Garden.
Revisit all brains after completion.
Uncover Milla's Secret.
Complete the Punchy Target mini-game.
Complete Meat Circus.
Complete The Asylum.
See yourself through the eyes of many others.
Complete Gloria’s Theater.
Achieve Rank 60.
Complete Waterloo World.
Find all Golden Helmets.
Complete Black Velvetopia.
Earn all Merit Badges.
Complete The Milkman Conspiracy.
Achieve Rank 40.
Complete Lungfishopolis.
Use the bacon. A lot.
Buy an item from the Camp Store on Christmas.
Complete Milla’s Dance Party.
Witness Maloof's transformation.
Achieve Rank 20.
Complete Sasha’s Shooting Gallery.
Discover the secret history of Whispering Rock.
Cook and consume two different kinds of roast in one sitting.
Read many bulletin board messages.
Complete Basic Braining.
Achieve Rank 80.
Achieve Rank 101.
Sort all Emotional Baggage.
Crack all Vaults.
Gather all Figments.
Redeem all 16 Scavenger Hunt Items.
Hear Vernon's Ghost Story.
Spy on Bobby's love life.
Give the Coach's speech on the stump.