When it's just too hot to be outside, just hop online and look for some dog-friendly shops to take your dog to. While most pet shops are inherently dog-friendly, you'd be surprised which chains and other local independents welcome dogs with open arms. Just don't forget the doggy bags!
It may only be an annual event, but Paws in the Pooloza is a great way to help benefit the Tri-County Animal Shelter and give your dog a chance to chill out in the pool, making it a cool and conscious choice!
Outside of the Barking Springs, Texas is home to quite a few dog parks that have water features. Do a bit of research and do a mini dog park tour if you find enough cool spots clustered together. You can always hit regular parks first, then cool off in ones with some water to cool off in.