I'm from Oklahoma, and although it is an unfortunate place socially and politically, it's pretty decent geographically and geologically. It is very flat around most of the state which is kind of boring, but it has some pretty great landscapes when you go looking for them. The biomes range from pine forest and rolling hills in the southeast, to prairie flatland/grasslands across the center of the state, to almost desert highlands in the northwest.

There are "mountains", but they're so old that they've been eroded basically flat, down to their granite cores--one of the contributing factors to Oklahoma' flatness, no doubt (not to mention it used to be under the sea, which is where our petroleum comes from). There are a few mesas and butes to the northwest, which stand out among the desert high plains, composed largely of red clay dirt and vibrant, sparkling gypsum/selenite/quartz cap rock.

Check out the "Glass Mountains". The thick layer of mineral deposit atop the these mesa structures would have been deposited during a great epoch of evaporation, increasing the concentration of minerals in the inland sea so greatly that they had no choice but to fall oit of solution--pretty wild.

There's also some sand dunes, but the ones in Colorado are way cooler.

Anyways.

Most of Oklahoma is, but as you get near the edges of the state you encounter some bumpy bits. Looks like Turner Falls is near the Texas border.

The northeastern bit is the beginning of the Ozark Plateau. Went to the very southeast corner one time and was surprised at the hills.

Now I live in the flattest state in America, Florida. It is freaky flat around here. Only waterfall I've seen is 3' tall.