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