In the early stages of planning a road trip that would take in - as a minimum - Yellowstone and Monument Valley. Advice please on the best time of the year - we thought September (2016) - and other places to visit. We have been to Death Valley, Zion, Grand Canyon, Sequoia,and Yosemite on a previous trip. Also which is the best US airport to fly into - we will fly from London, UK. We would probably stay at least a month. Thank you.