It's gonna be *hot* so if you're doing outdoor stuff aim for before 10 am and after 5 pm. At Zion, walking the Narrows is a great way to cool off and the water levels should be at their yearly low around then and monsoon season should be about done. But do pay attention to weather reports. Don't sleep on the Kolob Canyons section of the park along I-15 - super easy to get to, a few decent hikes, and hardly any people. Bryce is at a much higher elevation so it should be somewhat cooler. You probably don't need more than a day there unless you're intent on hiking all the trails. Red Canyon just east of Bryce is similarly spectacular and has some good trails. Outside those two parts, consider driving Route 12 - I haven't yet but it's supposed to be one of the most scenic drives in the U.S. Down that way check out Coral Pink Sand Dunes State Park - hike out on the dunes at sunset or rent an ATV and just play for a while. Goblin Valley State Park is a bit further afield but is a great day trip - the rock formations are singularly unique and you can kill a whole day wandering through them. There's also a great slot canyon hike (Wild Horse Canyon) that virtually anyone can do not far away. Another good option in the area is Cedar Breaks National Monument - fantastic scenery, ancient bristlecones, and a couple good hikes. One more - Valley of Fire State Park between Vegas and St. George is simply fantastic.