Beating the game without dying lets Marin escape the island. Depending on the version, she either has wings like an angel, or is turned into a seagull.
The picture you can take at the mysterious ghost's gravestone in the DX rerelease. After escorting the sad ghost to its former home, watching it cry from the nostalgia of its decrepit house, and then taking it back to its grave, it's nice to see that Link could finally give the poor fella a reason to smile and allow him to rest in peace.
How about the talk between Marin and Link on the beach? And all the fun things they can do together after that, like get their pictures taken or play the trendy game? It's disappointing that without a glitch or cheat you can't keep Marin in your party forever.