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