There's a nude beach near me with a remote section where sex goes on. I like to go there on a hot day, spread out my towel in a semi-secluded area just visible from the trail, and lie nude on my back reading a book -- or pretending to. My penis would be spongy but not hard.
People walk by every few minutes and most look in, then move on. But some find an excuse to linger. And when they do, and I find them interesting, I let my penis get a little bigger. This usually brings them in closer, pretending they are conversing or admiring the ocean. By this time I am hard. If they drop the pretense and look at me directly I will reach down and pull back my foreskin so my large tip is fully exposed. All the time I'll be looking at my book but secretly watching them.
If they are totally focused on my penis I will roll on my side facing them and give them the clearest possible view as I slowly stroke up and down. By this time somebody will usually say something like, "You're big," or, "You are so hard." I will reply by saying something like, "I like to be watched." If the watcher is a young guy, even if his gf is along, he'll usually be hard too by this time and and I'll invite him to share the towel.
I usually jerk off with a guy or for a woman to watch for about 20 minutes. Then I say something like, "Is it okay if I orgasm?" Believe me, that's something everybody likes to watch.
So I'll stroke slowly, all the way up and all the way down, fully exposing the big tip each time, and at the same time describe my inner process: "I can feel the sperm making its way through my loins and slowly up my penis. When it reaches the top that will trigger my orgasm."
I'm pretty noisy when I cum and as I get close I moan and gasp. Then as I lose control I say something like "Here it comes!" Short, sharp bursts of thick white sperm shoot from my cock in rapid succession, arcing into the sand. As the flow ebbs I stay hard and massage my tip for extended pleasure. There are murmurs of, "Great!" and "Good one!"
I'll say thanks and lie flat on my back to recover, utterly relaxed and swimming in a fog of post-orgasm euphoria. I cannot reach this intensity of climax unless at least one person is watching. :-D