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