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