I like the ghost in the graveyard idea. It could go something like:
<Villager near the graveyard> sees an strange old pile of bones near the edge of the graveyard. Should he:
A. Leave them alone. They could belong to some animal.
B. Bury them. They were obviously one of the island's former occupants, and should be treated with respect.
Ending One - The villager vanishes, and is never heard from again. But a new grave has appeared with his or her name on it.
Ending Two - (A pile of remains now becomes visible in the top left corner of the graveyard. The villager performs the act of burying the remains, just as if someone had died.) The villagers all experience a feeling of well-being as the ancient is laid to rest. Your tribe gains a level in spirituality! (as an additional or an alternate benefit, all villagers whose health isn't full could experience a boost in their health, and/or the sick could be healed. A tombstone appears with the name "Ancient" on it, with no age given.)
A good alternate spot for this event could be around the boulder!
