Has anyone ever taken a look at the moon? Or the Sun? Notice what shape they are?
Also, another fun way to prove the Earth is round is to take someone to a harbour, along with a telescope. Point the telescope out towards the water and watch a ship come in (preferably a sailing ship, or something with a tall mast). Notice how the ship comes into view? You see the top of the mast, and then a little more of it, then the deck, then the hull. If the world was flat you'd be able to see all of that ship when it was a tiny speck on the horizon.
Good times.
Obituary: A detective story I wrote