Things To Do
These arent really codes, but they are fun little secrets:

IN THE TOWN OF MELEE ISLAND

Get the fortune teller to tell you your future. Talk to the pirate in the white shirt and ask him about Loom in the Scumm Bar. Talk to the dog in the Scumm Bar. Ask the Important Looking Pirates about what is in Grog. Play with the rat for a while near the Men of Low Moral Fiber. Walk into the alley when you hear someone calling from it. Try to steal from the store several times. Look at the sign near the clock and look at the clock 3 times. Walk through all the doors in the store. Let the rat out of the prison. Let yourself drown when you get thrown off of the dock (be sure to save first!)

OUTSIDE THE TOWN

Look at the stump in the forest. Talk to Meathook before Elaine is kidnapped and ask him about his talking tattoo. Try to buy Stans ships that you cant afford.

ON THE PIRATE SHIP

Read the Captins log about Toothrot. Threaten the crew and then try to tell them what you think "Keelhaul" means.

ON MONKEY ISLAND

Walk to the far right of the heap o rocks (this one is funny.) Sink your ship with the primative art. Tell the natives to look behind them for a three-headed monkey twice. Look at the skeletons on poles outside of the giant monkey head. Keep getting captured by the natives and getting traped in the hut. Keep looking at the door each time.

ON THE GOST PIRATE SHIP

Board the ship before you use the eyeball necklace.