Articles and things that should be done or placed into a world.
Old cultures since the dawn of written time have seen pictures in the night sky and asked questions of them. Who they were, where did they come from, and why do they return? The earliest efforts to distinguish these nightly visitors and give them names and meanings dates back to before the Contention of Aborior. Those first observations were different than what is seen today but most still hold true to their original origins.
These are minor things that can be dropped in anywhere to add "narrative flavor", to make it more than just another road, field, forest or beach.
The Carycian calendar has been adopted by many countries since its invention by the Blue Monk, Frezzi Alberto, in the year 1. It evolved from the wonderment Alberto felt for the natural cycles and his attempts to understand and predict them.
Tobbacos, ingridients and mind altering herbs etc.
The lowly Undead cannot be a true replacement for a living well-motivated workforce. But Undead do not tire, they do not have moods (well none worth marking), they do not require pay or lunchbreaks, and can work 24/7, if properly controled. So until activists start to fight for Undead rights, they are an interesting option.
If you want to add a "supernatural" touch to your campaign, define the rules of magic and the universe. Make your players comfortable with those rules. Then your new supernatural creatures must then break those rules.