Here is the full list of all 21 Costume Quest achievements. It takes around 6-8 hours to unlock all of the achievements on Windows.
Find the mysterious visitor in the cliffs of Repugia.
Acquire the Eyeball costume.
Collect all Battle Stamps.
Use every Costume Ability in battle.
Collect all Creepy Treat Cards.
Beat the game.
Use the Robot Costume to Boost off of 15 different ramps.
Acquire the Pumpkin Costume.
Win all prizes in Bobbing for Apples.
Defeat Metxel in battle.
Win the Autumn Haven Mall Costume Contest.
Acquire 5 Costumes.
Get the jump on 10 different monsters by pail bashing them to initiate combat.
Defeat BoJonn in battle.
Collect a total of over 4,000 candies.
Defeat a monster in combat with a friend by your side.
Complete all Quests in the game.
Use the Pirate hook on 5 unique ziplines.
Complete all quests in Repugia.
Defeat Araxia in battle.
Play Costume Quest on Christmas.