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