i haven't camped much in north texas, so i'm not familiar with parks up there, but in the hill country area, which should meet your driving time, there are plenty of opportunities, though only a few for swimming. Inks lake would be good for swimming and has 7 miles of trails. McKinney falls will also satisfy your swimming requirement. Though with fall weather setting in i don't think i'd want to be caught in the cool waters of these palces

Enchanted Rock, Bastrop State Park, and Hill Country SNA would all be good choices, though they lack places to swim. In Oklahoma, i've heard good things about McGee Creek SP, and Talemina SP in those ancient Ouchita mountains is always beautiful.