OOOH!! I know! Post a video on the migratory habits of the North American homing pigeon!
Seriously though, if you don't know what to post, then you're already on the wrong path. YT creators fall into 2 categories: Those who post content they LOVE making and those posting to earn money from ad revenue.
I assume you are in the latter category, because you are asking what to post, so you're obviously not a passionate content creator. So, if you are making videos just for the money, then look at what others are doing to get ideas, but most importantly, stay on top of trends, especially with popular niche products, and post about them.
Oh, and be prepared for a LOT of work, as YT success doesn't come easy anymore and you are in for a VERY long road to success with all the competition out there.
Or start watching the pigeons...