Here is the full list of all 37 Psychonauts achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Achieve Rank 101.
Gather all Figments.
Crack all Vaults.
Read many bulletin board messages.
Uncover Milla's Secret.
Complete the Punchy Target mini-game.
Redeem all 16 Scavenger Hunt Items.
Revisit all brains after completion.
Find all Golden Helmets.
Complete Meat Circus.
Use the bacon. A lot.
Sort all Emotional Baggage.
Complete The Asylum.
Give the Coach's speech on the stump.
Introduce all Camp Kids to Mr. Pokeylope.
Re-brain the Children.
Complete Black Velvetopia.
Earn all Merit Badges.
Find Edgar's Secret Garden.
Complete Gloria’s Theater.
Achieve Rank 80.
Complete Waterloo World.
See yourself through the eyes of many others.
Complete The Milkman Conspiracy.
Achieve Rank 60.
Complete Lungfishopolis.
Spy on Bobby's love life.
Achieve Rank 40.
Discover the secret history of Whispering Rock.
Hear Vernon's Ghost Story.
Complete Milla’s Dance Party.
Witness Maloof's transformation.
Complete Sasha’s Shooting Gallery.
Achieve Rank 20.
Buy an item from the Camp Store on Christmas.
Cook and consume two different kinds of roast in one sitting.
Complete Basic Braining.