Here is the full list of all 39 Return to Monkey Island achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Answered One Hundred Trivia Questions Correctly
Answered Seventy-Five Trivia Questions Correctly
Answered Fifty Trivia Questions Correctly
Reached The End In 2 Hours Or Less
Answered Twenty-Five Trivia Questions Correctly
Rescued Wally From Monkey Island
Found All The Copies of ‘At The End Of The Plank’
Told Everyone On Mêlée Island That You Are Looking For The Secret
Shared Your Luck
Did Gullet A Favor
Decked Out The Sea Monkey II With Spooky Skulls
Braved The Scorched Alaska
Listed Every Mess On The First Swab’s Report
Got Hired With All Four Mop Heads
Attempted To Steal The Cook’s Mop
Offered To Donate To The Earthquake Relief
Ahoy! More Than Twenty Times
Explored The Entire Ocean Floor
Died For Real
Found Cogg Island
Returned To The World You Know
Performed The Ole Switcheroo With The Replica Flag
Waited Patiently To See Carla
Gave Stan His Toothbrush
Started Part Five
Collected More Than Twenty Trivia Cards
Let The Lumpsucker Escape
Became A Prize Chum
Started Part Four
Answered Ten Trivia Questions Correctly
Got To The Heart Of The Problem
Started Part Three
Swabbed The Hold Twenty Times
Started Part Two
Freed Otis
Thoroughly Examined All Of Wally’s Stock
Convinced Cobb To Tell You About LOOM
Started Part One
Fulfilled Your Restroom Obligations