Beach - yes, Galveston is the closest to Houston. There is parking along the seawall. It's not the white sand and clear blue water you see in the Caribbean, but it is sand and ocean. You will think Castaway Cay is much prettier. (When we took my daughter in September she opted to skip the beach - six years old - which was a surprise because she love swimming.)
San Antonio - Riverwalk - we always love eating at Boudro's. You can dine along the river or indoors, lunch is less expensive, and they do take reservations. Reviews are here.
Sea World is smaller than Orlando but still good. It now has the new water park integrated.
Thank you,
We usually go to the beach near Nice and it's not sand more rocks so even if it's not white sand I think we are all going to enjoy it.
According to the reviews Boudro's seems really nice.