Time capsules can be the story hook for a variety of #GammaWorld adventures featuring fake prophecies ("The Ancients will impart their knowledge to us on the chosen day...") and/or treasure hunts ("but we don't know where it's buried").
For example, a classic "weird local tradition" plot: One day a year, the villagers allow brave warriors to enter the nearby forbidden ruins and attempt to dig up the Ancient treasure that can only be opened on the sacred day. After some wacky #GammaWorld adventures in the ruins (including fights with rival treasure hunters), the PCs find one area with no signs of excavations. That's where an especially deadly monster or robot is (accidentally?) guarding the time capsule.
Or, flip that around for an "accidental outlaw" #GammaWorld plot: The player characters unwittingly open a time capsule *before* its prophesied date, making enemies of an entire settlement. Do they flee the area? If they're caught, do they talk their way out of punishment, face trial by combat, or buy their freedom by helping the locals with a different problem?
Or flip that idea around again: A #GammaWorld settlement's sacred time capsule is stolen a few days before it's scheduled to be opened. The settlement's leaders cover up the theft, then hire player characters to secretly retrieve the capsule and return it to its rightful place before the general populace learns of the heretical crime.
Of course, since nothing is for certain in the #GammaWorld, the contents of a sacred time capsule might be anything: Useful artifacts, useless knickknacks, or misunderstood portents for the PCs' next adventure -- "The Ancients' scroll confirms that Dewey did indeed defeat Truman! We must declare war on the blasphemous Trumanite tribe immediately!"