There are different types of entries with which we can fill an imagined history. This list isn’t exhaustive but can provide inspiration.
In some story worlds, the gods keep to themselves, but in others they interfere with life and cause events. This could be fathering children with members of mortal species, resulting in monsters or demi-gods. They might have sacred places that are built or destroyed. Perhaps most fun is that gods sometimes have very powerful magic items that fall into the wrong hands, causing events with long-term consequences. This can include the invention of new species or monsters that proliferate. A god can even be killed or otherwise inhibited (imprisoned) due to some event, whether they acted badly and were punished by the gods, or because a mortal somehow did something to them. Maybe some of these incidents are celebrated or otherwise noted, like an anniversary, by the species.
Many technological events are possible, especially in SF. These include failed or successful missions, rocket/satellite/ship launches, explosions, discoveries, weapons tests, and first contact with other aliens/species. The latter is especially important; the longer the history between two species, the greater the odds of conflict and familiarity breeding contempt. Or those who’ve been hostile can learn the other isn’t so bad, possibly due to help needed against a mutual threat. Disasters of technology, like America’s space shuttle tragedies, often spur innovation and memorial; a main character can be a descendant of someone lost in one. Classes of vessels might also be invented, used, and then retired. Another event is the discovery of special ore needed by ship engines or to forge metal in fantasy, or even the creation of famous items. Ships, fleets, and items can also disappear, be destroyed, or be captured.
For fantasy worlds and some SF, no history is complete without the supernatural. Phenomena must start or be discovered, and possibly wreak havoc, and then be neutralized. We might have famous expeditions to deal with somewhere or something. Magic (in general or specific spells) can be discovered, expanded, and proliferate or be squashed in one region or sovereign power. Famous practitioners or victims will come and go. This is true of items, whether armor, weapons, scrolls, potions, or jewelry. Perhaps a useful material is discovered or exploited into nonexistence (or restricted by law). Monsters might result from events, too. Did something of the gods fall into mortal hands? Celestial events like a conjunction, eclipse, or comet can be assigned supernatural significance.
The Rise and Fall of Sovereign Powers
No sovereign power lasts forever or retains the same government. The bigger events, such as the power signs an important treaty, or collapses can be noted here, but most internally relevant events can be kept in that power’s file. How a kingdom came to be doesn’t really require an explanation in our world history, and few will question this. However, its demise benefits from an explanation (see Chapter 5, “Creating a Sovereign Power”); these include being conquered, revolution, and a coup. Its fall will impact neighbors, but we might not need to comment on this unless an empire has fallen, as other powers that fell under its control will enter a period of instability.