Ok, so full disclosure, I believe the earth is round, but I want to have an informed discussion about what I think is the simplest and easiest way to prove that it is so. This is something that can be proven with the naked eye.
The moon always shows the same side of itself to earth, no matter where you are. This is something that is obvious simply by looking at it in the sky with the naked eye. You can spot the same creators and features of the lunar surface anywhere in the world. The only difference in how we see it between the northern and southern hemispheres is that they are upside down of each other. So in the southern hemisphere, the moon looks the same except that it is upside down from how it looks in the Northern Hemisphere.
How is that explainable on the flat earth model? Since the flat earth model involves the moon circling on top of a flat surface?