Really?? :h Texas in July?? :E
HOT HOT HOT
The lows at night can be what the high temps in Detroit are in the summer.
But if you must go:
Beaches in TX aren't anything to write home about. Very fine sand, almost mud along the LA border and they get sandier as you head south toward Mexico. Beaches aren't too crowded in the summer as it is so hot. Even the water is like a warm bathtub, not very refreshing.
As noted, San Antonio and the hill country would be on option. Visit indoor museums, water parks, float down the rivers. Visit places like the Alamo in the morning then find a cool place to hunker down during the heat of the day.
Another option would be the Ozarks or Ouachitas in Arkansas. Since they're a little higher in elevation, it's a little cooler there by 4 or 5 degrees. Plus, they're on the way from Detroit. Lake Ouachita is amazingly clear with a variety of camping options including COE and an SP. Besides the usual outdoor activities, there are interesting museums in Hot Springs and Little Rock and a great free art museum in Bentonville (Crystal Bridges). Plenty of little streams to play in.