Oh, Utah, one of our favorite places...  The National Parks of Southern Utah - Zion, Bryce, Arches, Canyonlands, and Capitol Reef - are all amazing and quite different from one another.  We were there in July/August (2015)
Colorado/Utah: beautiful parks, wonderful memories
and most places were crowded as you'd expect.  September might be a cooler, less busy time to visit.
In Zion, The Narrows, the hike up the Virgin River between the canyon walls, was a highlight.   Also, Angel's Landing hike is spectacular,  but that might be restricted now.
Delicate Arch hike at Arches NP is an iconic site to visit.  Moab is an interesting town between Arches and Canyonlands.  Great campground in Capitol Reef near Fruita, UT.
Also, we stayed at a great state park, Goblin Valley, in Green River, UT.
You just can't make a bad choice in Utah is what I'm trying to say I guess.  Give yourself plenty if time to spend exploring the variety of landscapes that each place offers.  Enjoy!