Unlock access to the following uncharted islands by performing the corresponding tasks:

ISLAND - HOW TO UNLOCK
Cliche Island - Collect the five pieces of Captain Taint's map. Start the quest to help him get his treasure. He tells you it is on Cliche Island and that a Mutinous Captain may have it. You can also find Mutinous Pirates with the captain on X Island. Just west of X island is Chicken Island, which has the parrot feather required for a different quest. Start at the dock at Chicken Island, then move straight west until you see a small island slightly to the north. Dock there to find Cliche Island and a treasure chest.
Hot Island - Move south from the western side of Spider Island.
Skull Island - After invading the North Pole, get on your ship and travel south from the dock. Look for a few rocks sticking out of the water and a spear with a skull on it.

ACHIEVEMENT - HOW TO UNLOCK
A Tale of Two Endings - Watch both game endings.
A True Hero - Reach level 20.
Death of a Penguin - Kill the Evil Nun.
Dispenser of Justice - Kill 3000 Enemies.
Frozen Justice - Kill the Taking Tree.
Konichiwa - Learn to Speak Japanese.
Mystic Transport - Discover all 44 Teleporting Outhouses.
Orque Impostor - Convince an Orque Guard you are an Orque.
Primp V.I.P. - See all 20 of Madame Primp's Outfits.
Raise Your Mast - Go Sailing in a Pirate Ship.
Ruin Christmas - Kill Santa Claus.
To Serve Man - Kill wortten.
Well Educated - Get an Engineering Degree.