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