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