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