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