What about Coral Pink Sand dunes? It is in southern Utah. I am not sure what the weather is like in mid to late summer though. I was there last year in May, and it was beautiful. It is really close to Zion National Park.
I am really new to Arizona, and the surrounding area, but thought I would throw that out there, because I have been there before, and it was a lot of fun. Not exactly sure if that is what you guys are looking for or not.
Or maybe Lake Powell, It is North AZ and South Utah. I personally have never been there, but have had an interest to check it out.