I am a native of Oklahoma, but have lived in Missouri 25 years. I am also south of Springfield which is an hours drive from where I live. Missouri for the most part is not flat. Missouri has the Ozark Mountains and in the foot hills of the Boston Mountains of NW Arkansas. These mountain ranges are not of the scope of the Rocky Mountains, but nonetheless certainly not flatlands. There are lots of equestrian trails in some of the Conservation areas and logging roads. There is at Emineince,MO one of the "hot spots" for camping and horse trails. It is a very popular place for many,many horse campers and trail riders. There is a wide range of disciplines for the horseperson. For the Dresssage set there is the Ozark Dressage Society(ODS).
As others have said the Winter weather has many varibles also. The deepest snows have only been about or just under 2 feet deep and never last more than a few days. Some Winters we have no snow or ice at all like this past Winter. The Summers can get hot and humid, but not unbearable.
Hannibal,MO is the birthplace of Mark Twain aka Samuel Clemmens who wrote "Tom Sawyer and Huck Finn". It is on the Mississippi River.