A.S.
There is so much to do within 6-8 hours of DFW. San Antonio - Sea World, Alamo, Riverwalk. Houston/Galveston - NASA, Beach, Schlitterbahn (they have an indoor part in winter). Glen Rose - Camping, hiking, dino bones. Grandbury - lake, camping, water sports, horseback riding. Fredericksburg - Enchanted Rock, antiques. San Marcos - Aquarena Springs.
What are some of your top activities to do? You could even venture up into Oklahoma (Turner Falls - camping, hiking) or even into Louisiana with those limitations :) It is almost never-ending.