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