On the recommendation of the rangers at the visitors center , we concentrated on Delicate Arch, Landscape Arch and Windows. Here's some thoughts!
Delicate Arch - we started this fairly strenuous hike (uphill for 45 - 60 minutes - an option is the less strenuous viewpoint hike) at 6p. The light was terrific, we got great photos and there were maybe 30 - 40 people there getting ready for sunset. As we hiked down at 7:45 there were hoards of people heading up for the sunset picture so you need to decide what is important to you - viewing with the few or sunset with the hundreds!
Landscape, Pine Tree & Tunnel Arches - We were there at 7:45a on a Sunday and not only was it perfect light, there were very few people on the trails!
Windows - we enjoyed this area as well but be warned - by 10a the buses and the non stop line of cars were snaking in to the park.
Be an early bird - avoid the crowds - get incredible photos and avoid the the heat of day!
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