I live in Austin and have to deal with this about four months each year! For myself, I get out and exercise in the morning, before it gets too hot. Whenever I go on a long hike, I drink Gatorade - I didn't do this on a hike about 10 years ago and I got heat exhaustion.
For DD (who doesn't have CF) we go to a lot of sprinkler parks and pools, and we go to indoor gyms. We just spent a week at the beach and she loved it - I wish we lived closer to the ocean!
I can't wait for fall!