The middle of the country is really lacking in big parks. The only thing I could come up with are the Buffalo River (Arkansas) and the Ozark River (Missouri) areas. If you like floating down rivers. Other than those the only others are GSMNP and Shenandoah. Looks like the rest are historic sites of one type or the other.
There may be some great state parks tho in Arkansas and Missouri you might want to look at their state park sites. Texas certainly has some very good state parks but that's a bit out of the way for both of you.
I just spotted this:
America's state parks site
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