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