What I want to avoid is situations where (for example) the world is static and the sun settles on a mountain top at the end of the day and extinguishes itself (as in KRussellB's delightful idea above) and then a player says: "so how come there are seasons?"
Thanks for the shout-out.

To me, that question is honestly why I love having the mythical be real in campaigns. It creates this really fun domino effect of explanations. It's like how in Greek mythology, all these myths are tied together, all working under this theme of scheming, jealous gods.
Asking a question about the seasons opens up now the opportunity for more mythology... and the opportunity for adventure paths, magic items, monsters, etc. Let's say I'm continuing with my Sun King World above... There were once four elemental rulers who divided the land. To the north, all was cold. To the south, all was hot. To the west, spring rains flooded the land. To the east, fall harvests overgrew each other into mountains of vegetables. There was some terrible war between the four rulers, at the end of which they decided to divide up the year into four seasons, each of which would be ruled by one of the elemental kings.
The players might hear this and roll their eyes. But what if they find a dungeon in the east carved into a giant, petrified pumpkin? Or the remains of a once-underwater city to the west, abandoned millennia ago when the flooding rains stopped? Or they found the Horn of the Northern King, which instantly turned the weather to a snowstorm?
Then again, what if one of the elemental kings was trying to take his land back?
OR: What if one of the elemental kings died? Now there's no Autumn! What does that do to the harvest, to the crops? How can they restore an Autumn King and bring Fall back to the year?