What are you calling the Midwest? Including the Great Lakes? Great Plains? Southern Plains?
One area in Wisconsin would be the Dells, my favorite park in the region is Devil's Lake SP. It is a Great Lakes state, but more a midwest part of Wisconsin than a Great Lakes part, being a short drive from northeast Iowa.
You have good access to areas at or near shore of Lake Michigan: Indiana Dunes, Warren Dunes, Grand Haven. Or on up to the NW corner of the Michigan LP, Petoskey to Ludington, where the glacial processes that created Grand Traverse filled the area with some long, deep lakes. Much of the waterfront property has been occupied in summer by wealthy families from Chicago and Detroit areas, but there are state parks (Interlochen a favorite of mine) and commercial resorts as well.
If you include southern Illinois, Kentucky, Tennessee, in your Midwest, then Land Between the Lakes where the property confiscated when lower Tennessee and Cumberland Rivers were dammed has now been turned into public recreation area, both Federal and state.
But most regional maps now put the name Midwest on what is between the Mississippi and the Rockies, and usually north of Texas. Oklahoma can be either a Midwestern or a Southern state, depending on who is making the map
In your closest part of Missouri (that's usually considered Midwest today) you'll find family fun along the Mississippi River in Hannibal, and water recreation on Mark Twain Lake. Deeper into Missouri, most of the river systems feeding into the lower Missouri have been dammed for flood control, forming large reservoirs like Lake of the Ozarks, which is a huge resort area, water sports and tourist trap fun in addition to the state parks.
There is another complex on the Arkansas River and its tributaries, resort and retirement areas in SW Missouri and NW Arkansas from Springfield/Branson east through the Bentonville/Fayetteville area to Sallisaw, Oklahoma. Resort towns, a kind of "country" Las Vegas, lots of camping on the water in state parks and Corps of Engineer recreational access facilities.
If looking for mountains instead of rivers, the Ozark Mountains will be found in Arkansas between the Missouri line and Arkansas River/I-40, NW of Conway. The rest of what is called the Ozarks is a deeply carved low plateau, but common south off of the flat cornfields, those rocky river valleys make the plateau look like mountains to flatlanders.
For mountain ranges, there are the Ouachita mountains in SW Arkansas and SE Oklahoma, with camping in forests and valleys, scenic drives running on the ridges, looking like a little bit of the southern Appalachians.
Go deeper into Oklahoma, there are more reservoirs on the Arkansas and Illinois rivers and their tributaries, mostly east of I-35. Grand Lake of the Cherokees, Tenkiller Ferry Lake, Lake Eufala being the largest, but many more. CoE Tulsa District manages 29 reservoirs in Oklahoma, Kansas, and N. Texas, as well as Oklahoma's only natural lake, with something more than 200 parks. There is RV camping at another 50 state parks in Oklahoma, most on reservoirs or rivers, and another 25 state parks in Kansas, many on Corps reservoirs or waterways.
I can't help you with Iowa, Minnesota and the Dakotas, there are enough camping places within a day's drive in Texas, Oklahoma, Missouri, Arkansas and Kansas that I don't get much further when my purpose is just finding a place to camp.
Tom Test
Itasca Spirit 29B