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