If you like outdoors and landscapes, July is not the best time, and Houston isn't exactly known for landscapes. The cool things in Houston would be its people and cheap and delicious food from all around the world. There are a few nice museums (Menil- free and in the 1,000 Place list, Fine Arts Museum- free on Thursdays thanks to Shell), but you said you're not really into that. It is also known for performing arts, if you are into that. I really like to picnic and watch free performances at Miller Outdoor Theatre in Hermann Park
http://milleroutdoortheatre.com/events/month/. There are also some quirkier museums/sights in Houston, like the president statues (never actually been so I don't know the name) and things affiliated with the Orange Show Foundation: Art Car Museum, Beer Can House, Orange Show.
Austin has a lot more outdoor sights and activities, even within the city: Lady Bird Lake (formerly known as Town Lake), Barton Springs, Barton Creek Greenbelt, etc. Barton Creek Greenbelt may be dry, but there are some cool spots like Sculpture Falls and Twin Falls. As you go out west there are even more county and state parks with scenic spots. I've done Austin--> Hamilton Pool --> Enchanted Rock for sunset --> Austin in one long day before and have also just ended in Fredicksburg before.