Point Supreme at Cedar Breaks is a lovely little CG for dry camping (at 10,000ft elevation). Last summer they had the best maintained state/federal bathrooms of any place we visited. Not too much hiking, but you can get some miles in mostly along the rim.
If you can't get a space at Point Supreme, Duck Creek has much more room (but is busier and noisier).
Snow Canyon State Park just outside St George is peppered with hiking trails, including some over petrified sand dunes.
Valley of Fire SP is first-come, first-served on camping so don't try the weekends unless you plan to get their very early and catch someone leaving.
With Zion, check map locations for the non-park CG's as folks as far as 30 miles away use Zion in their name. And this year don't plan on visiting Kolob Reservoir (road out of Virgin) as they have a major project causing closures.
I haven't stayed at Coral Pink Sand Dunes, but the area is other worldly.
2013 Heartland North Trail 22 FBS Caliber Edition
2013 Ford Expedition EL with Tow Package