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