I'm currently planning a road trip with my 7 year old daughter to see some friends who are renting a condo on the beach near Corpus. Since my husband isn't going, I decided to stop one night on the way down and back to break up the drive. I think I've decided on San Antonio at the Inn on the Riverwalk - http://innontheriverwalksa.com. They're offering a promo - $25 per night off your stay (up to $50) if you show a gas receipt. I figured I can get one of the cheapest rooms for around $75, which includes breakfast and if you like their Facebook page, you can get free breakfast delivered to your room each morning. They say they're not too far from the commercial part of the Riverwalk and an easy drive to area attractions. There are Seaworld packages for 4 people (look on the Seaworld San Antonio site). We're also going to stop in New Braunfels and visit the caves and children's museum. Someone told me about Salado which would be on the drive down. It has an exotic animal petting farm (looks like it's free to visit).
We went to Fossil Rim (south of Fort Worth) a few years ago - once as a day trip and once as an overnight trip. Overnight is better - less rushed. You can also go to the dinosaur park. Another time, we spent a night in Fort Worth, which my daughter loved. We didn't get out of the stockyards during the visit even though we wanted to visit the zoo and some museums. You could make it a 2-3 day vacation.