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