If there is no one to open the box in a hundred years, Who cares... But a water activated battery and some stored water and some instructions to "Pour Water Here to Activate" should work... All sealed in an inert gas? Nitrogen?, Neon, Argon to avoid corrosion. If the water contained a mild acid, Zinc and copper would serve well and I'm sure that there are better methods than what 2 minutes consideration gave me.
Bob