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