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