I don't know about the camping sites, but Cedar Hill SP has some of the most popular mountain biking trails in north Texas,
http://www.dorba.org/trails/trail_guide/chsp.html, which also make great hiking trails. Cedar Ridge Preserve,
http://www.audubondallas.org/cedarridge.html, (formerly known as the Dallas Nature Center) is immediately across the road from the state park and has some of the best hiking trails in north Texas. They are much hillier than most other trails in the area and the limestone of some of the trails is reminiscent of the Texas Hill Country.