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