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