Tent or Camper/RV? I'd check some of the County/Lake/River Authority Parks in addition to the State Parks. We've had more luck with that and private campgrounds over the state parks on availability. If you are looking to stay close I-35, there are a couple of lakes outside Temple/Belton that may have camping. Like someone else mentioned, 281 will have more to offer, but also starts getting you pretty far West. I guess it all depends on how scenic or how far off the beaten path you want to go. If you give me a little more info, I could give some more specific recommendations, but be warned, they'll probably involve some sort of geologic interest.
Edit: Must have book, IMO.