Hiking
We've hiked nude for quite a few miles in the Eagle Cap Wilderness, outside of La Grande. We hit the sweet spot in September after schools are back in session but before hunting season. We also hiked midweek. We ran into no one else, though I suspect if we had seen anyone they would have been very tolerant. One hike was about 9 miles, the other was about 12 miles; both were out-and-back rather than a loop, so especially on the way out, we were pretty well assured of not running into anyone since the parking lot was empty.
In general, if you find a trailhead empty of cars for a trail that has no other nearby entry point, and you do an out-and-back rather than a loop hike past other trailheads, you can pretty much hike nude anywhere out of site of dwellings and businesses. Weekdays are of course a better bet than weekends, and the further you are from a population centre the better your chances are.