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