Calendar and astrology development. The true power fantasy of fantasy worldbuilding is devising a world with a consistent number of days in a month...
Sounds fiddly lol. Not to knock it, everyone has their thing, I just don't think I've heard that one before.
i always loved the shire calendar in the indices of the lord of the rings. it's just so neat and tidy.
I love designing the geology. Mountain ranges, oceans, plate tectonics (when applicable), etc.
i always like thinking about the everyday cultural stuff. art forms, foods, music, attire, and so on.
I'd probably say mundane ergonomics (eg.: how do your species use chairs, or if doors have knobs or bars) and general interpersonal processes: how does one find a mate, or raise a child in the setting, how are liars and cheaters treated, etc.
