interesting choice of place to start this thread
each to its own when it comes to niches on instagram or anywhere else
i think you're maybe onto something though, when you said that you find fitness boring and you'd prefer something else, because you'll be more successful with something what you do with passion
there are quite a few guides on bhw about how to select a niche, some of those general guidelines can be applied to instagram too, so maybe start with that
what else?
study ig's demography a bit, a fast rundown: 2/3rd of the ig users are women, mostly young females under the age of 35, so you can concentrate on this majority and select a niche like fashion, beauty, nutrition, makeup, travel, cooking, jewelry, crafting, hair, nails, skincare etc. or you can concentrate on the minority and do something different, it's up to you
study the popular hashtags, so you can get a better picture what is popular there, but at the same time not always the biggest/most popular niches are the best, instagram should be big enough by now for us to be able to be successful in more obscure niches with little to no competition