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