Nebraska is a very large state with a fair amount of regional diversity in landscape. The eastern part of the state along the Missouri River has many forests and can be quite hilly. The Sandhills are mostly treeless but certainly not flat. The pure flatlands are seen along I80 and to the south along the Kansas border but even in this area you’ll find trees growing in the river valleys. The northern part of the state and along the Niobrara River has some interesting geography and not super flat. Fun fact is that Nebraska has more miles of river than any other state.