I use Claude.ai rather than Chat GPT, but it works similarly.
I like to brainstorm via extrapolation and casting it in various roles. For example,I posited a situation in which a massive extraterrestrial space ship appeared in orbit of Jupiter and was seen via a probe. I asked it to write a news article about the event, then a Reddit post about the article, them a reply to that post, then a news article about the Reddit tiff, then a news article about a press conference about the event, and so on. I don't know how much ChatGPT remembers, but Claude remembers the whole conversation so you can refer back to earlier elements. It also responds to requests to rewrite and change aspects.
It doesn't produce anything publishable, but the result is definitely a solid outline for a hand crafted work.
Aside from that, I like asking it to describe a new monster, then putting that description in an image generator and asking Claude to make a statblock for the creature at specific CRs.