J.W.
Houston has so many fun things to do...as does Galveston and Kemah which are not far at all!!
Our kids (9+6 at the time) LOVED the space center. We were there for 7 hours and they could have stayed more if wasn't closing.
FYI - there are toll roads in Houston, they work with the Dallas Toll Tags..the charges will appear on your tags from here.
Have fun!!