September
Hello Rick, I don't know of any trails in the RMNP that are open for nude hiking. There are a lot of trails, and maybe you might find one which is the least used. I hike in an area between Loveland and Estes Park on a not well used trail. Then you may want to check out the Group - C.O. in Co . Mick has a 30 acre private land which he has open for nudist hikers. I been there earlier this week and it was great.
Marc
In more normal times, I've hiked several RMNP trails naked. That said, with so many people unemployed and out in nature, it's difficult to find trails where naked hiking will not get you in trouble. You could try the Cub Lake trail from Moraine Park or possibly the trail on the east side of Arapahoe Bay. In any case as others have said, the Park may not be your best choice...
From a legal perspective, don't forget that RMNP is a National Park, not a National Forest. I don't recommend it, if you can go into the National Forest instead. There are differences from a legal perspective when it comes to a Park vs. a Forest.