For July in Southern Utah, use altitude as your friend. As others have mentioned daytime highs in Zion will likely be 100+. But Bryce will be cooler and more comfortable.
Other high-altitude options in my part of the state include Cedar Breaks National Monument (a mini-Bryce-like formation), Pine Valley Mountain, Beaver, Cedar City, Duck Creek. And from some of these locations you can visit some fun State Parks like Kodachrome Basin and Coral Pink Sand Dunes.
You'd also be within easy reach for the North Rim of the Grand Canyon (approx 8,000 ft elevation).
Be aware that most of the Nevada State Parks heading back towards California don't take reservations (first-come, first-served) so it's hard to stay at them unless you have a lot of flexibility.