Chicago to Austin

Rt66...Prison in Joliet where they cut Jake Blue's loose, Lincoln Museum, Lincoln House, Ted Drewes frozen custard St. Louis, Lamberts Restaurant Sikeston and Branson MO, Hot Springs AR. Stop at the state lines welcome centers.

St Louis is right on the way. Cards game, Grant's Farm, and a few state parks are an hour away (either side of river). Meramac caverns (I've driven by but never been there)

Lake of the Ozarks in central MO

Springfield, Branson MO and Silver Dollar City.

KC, nice camp lake N of town, Smithville Lake, 30 minute drive. Arthur Bryant's BBQ.

Lake Arcadia Edmond OK camping. 20 minutes to OKC

Lake Texhoma or Lake Murray (S of Ardmore).

Wichita Mtns in Lawton Okla. Campsite + Buffalo. Meers store burgers.

DFW, too much to list, several lakes and campgrounds.

All of this is on the way (more or less)

Ozarks are a good suggestion. Land Between the Lakes (KY/TN) is a great place too - a farther-east alternative to the Ozarks, with some very nice (and popular) CGs. With a tent they'll have some flexibility in sites, which is helpful. St. Louis's arch & zoo are terrific; Graceland in Memphis is a kick. Just a few ideas. Wherever they go, it's likely to be hot.