That's a tough question to answer! I'd think of two things: one the level and two perspective.
All these people go out and use zoom lenses to look across a body of water and find that they can see all the way across a lake or far out into the ocean. Of course, if the Earth was curved, then they shouldn't be able to see that far. In regards to the level, the other thing was the shear drop of it. Our Earth is supposed to have a diameter of about 8 thousand miles and a circumference of about 25 thousand miles. So, traveling from the North Pole to the Equator should be about 6 thousand miles and the traveler should experience a drop of about 4 thousand miles or a drop of 8 inches per foot traveled. We of course experience no such thing! It would be like living on a staircase! Now I'm told that I shouldn't notice that, because I'm like an ant on a beach ball. Well, I've traveled to the Equator. Why didn't I fall off the ball? I'm told that's because of Gravity. O.k., if Gravity is real, then a 100 lbs of lead should attract more dust, then a newspaper. However, in my experience, a newspaper attracts just as much dust as 100 lbs of lead or even 1000 lbs. The idea that Gravity can hold a cruise ship to the carribbean, but I can easily walk around that ship seems fantastic (like fantasy).
The second thing that really got me was Perspective. It was pointed out to me that a telephone pole not far away looked like it was only a few inches tall. Of course, the telephone pole handn't got any shorter; it was just my perspective. The telephone pole I stood next to still looked very tall! So, Perspective created an illusion of roundness or curvature.