I would love a small, portable sketchbook that Zois could keep in their inventory and use almost anywhere.
At the moment, creative activities such as painting are tied to easels. A sketchbook would allow artistic Zois to draw more naturally while relaxing on a couch, sitting on a bed or while being outdoors.
Possible interactions could include:
Sketch freely — create a random drawing influenced by the Zoi’s art skill and mood (similar to the “regular” painting system)
Sketch the surroundings — draw the nearby scenery, a building, an object, or another Zoi. (Per example by clicking on an object nearby. Scenery implementation may be more difficult though)
Sketch from a photo — select a saved Photo Mode screenshot and let the Zoi use it as a reference.
Practice drawing — improve the relevant creative skill without needing a full easel setup.
The finished sketches could become actual items or pages that players can:
- keep inside the sketchbook
- tear out and frame
- pin to a wall or corkboard
- place on desks and shelves
- gift to another Zoi
- sell through the Pocket Market
It would also give Photo Mode screenshots another purpose. Instead of remaining only in the gallery, important moments could become drawings made by the Zois themselves.
This could be a relatively small feature, but it would add more portable creativity, relaxed activities, storytelling, and personal keepsakes to everyday gameplay. 
I would especially love to see Zois sketching comfortably on beds and couches instead of every creative activity requiring a dedicated workstation.
Edit: Of course this can be applied to a tablet/iPad for drawing. @LadyDianaDi was so kind to mention it and reminding me.
Here are also some of my other topics, which would go along well with this feature:
