Skinnydipped in a pool at a friend's friend's house on Capitol Hill after a night of margaritas. I'm surprised that I was the first one to get naked, mainly because I knew how much it would suck to have wet underwear.
Used to ride my bike into the woods near the reservoir in college in the middle of a hot day and get naked. I usually jerked off, too. I also did it on the trail along the canal towards the reservoir at night, sometimes riding naked for a ways.
I've been to a few public baths in Germany, too, a couple of which with good-sized nude sections. The last one went all-naked promptly at 6 PM Tuesdays and Thursdays (and, I've been told, all day Wednesdays). I was there past 6, made my way to one of the outdoor pools, parked myself along the edge in the recliner-style layout near a couple women (actually, it was kinda crowded, and this was the most open spot), and waited for the bubbles to start. Once they did, and because I had been naked for several hours already, I couldn't hold back anymore and covertly shot my load, the women a mere arm's reach away and completely unaware.
Oh, and on the roof of my building, at night and in midday. Usually with an orgasm, too.
One more -- went for a nude ride on my bike around the neighborhood one warm, rainy summer night when the power went out. Didn't turn out any worse than the other times, either.. lol