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