Arches
if you haven't been to Arches, that would be on the top of my list, there are also Canyonlands, Capitol Reef NP, etc, Utah has some of the best NPs.
Smoky Mountains could also be a choice, I'm in the middle of writing a blog of my trip there last year:
http://blog.wenxuecity.com/myblog/65015/201404/10036.html