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