I think in 'Sundays Day of Stories', the stories might include the simple joys of a Sunday. For example, a group of friends getting together on a Sunday afternoon to play games or just chat. It could also be about the local Sunday markets, with all the unique items and the interactions between the sellers and buyers.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.
A simple Nezuko and Zenitsu fanfiction could be that they go on a journey together. Along the way, they encounter other demon slayers who are initially skeptical of Nezuko. But Zenitsu stands up for her, explaining that she's different. This shows the depth of their friendship and how Zenitsu has come to trust Nezuko completely.