Lots of the central-eastern portion of the state is the beautiful Flint Hills area, which is obviously… hilly. Much of I-70 (the main route across the state east-west) features long horizons but lots of low hills in the distance. The West, central, and southwestern parts have the slowly but consistently rolling, low hills of something like the film The Wind or the famous painting Christina’s World (the one with the girl on late summer or early fall green-and-tan grass, looking away from the viewer toward a white farmhouse up a low hill). They’re all much flatter than much of the country, but they’re not flat flat. They’re not Central Nevada flat.

South-central and southeastern Kansas are about that flat. Though with somewhat more trees.

[edit] I mean the remake of The Wind from a few years ago. I’ve never seen the original Lillian Gish film.

[edit edit] Further, those kinds of super-long views like you mentioned are actually kind of hard to come by on the really flat parts of Kansas. You need a little elevation (for the spot you're at) to get over obstructions like bushes, trees, barns, corn, et c, and that's what delivers those impressively-long views that vanish at the horizon in other parts of the state. That's why you need to find one of the (many) very-straight rural roads in the really flat parts of KS to see the "flat all the way to the horizon" effect. Or stand on the roof of a barn or something like that.