Where at in Texas?
If Austin:
I'd suggest
-Town lake, maybe paddle board in the morning before it gets hot then move over to Barton springs pool to cool off
-If you prefer more of a hike then I would go down one of the many paths leading to or from barton springs near the green belt
-Mount Bonnell is a good quick trip and not really a hike, more like a walk up some stairs. Don't let the name fool you. Its a hill
-If you want to see a nice sunset go to the Oasis above Lake Travis (Don't go expecting great food though)
-Visit one of the many local breweries (I went to 3 yesterday all within 3 miles of each other)
-Walk down south congress to get a t shirt and enjoy the view looking up at the capital and downtown
-I'd go enjoy some jazz at the elephant room to enjoy some night life downtown
-Go enjoy some BBQ, there are so many in town just pick one with a semi short waiting line
-Listen to some live music
Houston:
-Stay somewhere near the rail if you stay downtown and move down to the museum district for good food and some great art
-If you stay downtown realized that downtown is all business during the day and quite dead after work hours. Of course, bars are open but its nothing like Austin or Dallas downtown
-The zoo if pretty good if thats your thing
-Visit one of the many large parks
-Houston has hands down the best food in Texas. Its such a diverse city you can find anything and everything
-Grab drinks in montrose neighborhood, enjoy some live music
-eat more great food
-visit NASA and see the real control center for the Apollo missions
-With all of the national sports teams present and festivals that go on, see if you can find some event that you enjoy
-this time of year, its hot and humid. Good luck bro
San Antonio
-Stay near the river walk and check out all the tourist destinations
-I would honestly go during a 1st friday weekend. Its a mini festival full of live music, arts, craft beer the first friday of every month in a district just south of downtown
-Its going to be hot
-try some great Mexican food
-visit the missions
-wonder around downtown for some great night life and live music
-if you're traveling with family consider going to schlitterbahn in new Braunfels (about 30-40 min north of downtown. Its a very large water park which in the summer time will be much needed to escape the heat
Dallas/El Paso, I don't have as many recs. Feel free to message if you have any questions. Each city in Texas is very different and have their pros/cons. Be ready for heat in the summer. The other day my car thermometer was already reading 103F.