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