There is a challenge that is called "100 days creativity challenge". You're supposed to create a given sim every day (of course you can skip days when you can't play or you're not up to it).
I would play it this way:
- create the given sim, play with him/her during that day.
- tomorrow create another one and put him/her in the same save. When you play this last one, you should make him/her interact with the one you created the day before, with stories that relate to their cas characteristics.
- Repeat it for 100 days.