Lake Mineral Wells State Park, west of the metroplex, is a nice area. It has been several years since I was last there, but at the time it was very enjoyable. A rock climbing area is available. Palo Duro Canyon, The Grand Canyon of Texas, is a possibility to the north, although the hanging may be problematic in the area. Iím not sure if the trees in the area are substantial enough to support a hammock. During the summer you should be able to do some amazing star gazing from down in the canyon.