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