Sun tan oil
There are times when I've been to a beach with people I just met at the resort. We hardly know each other. But there is one thing for sure - we all need sun protection. And we can't put it on our own backs. So I offer to oil the back of my new friend, starting with the places he/she can't reach. I work my way down to their buttocks, massaging it well in. If there is no objection (and often there isn't) I ask them to turn round. Starting with the chest I work my way down. By this time I get a good clue if they are receptive, allowing me to massage it into their genitals. I think you can guess what happens next.
I was out hiking with 5 other nude men today. One has a concern about the size of his penis. But, after he'd rubbed sunblock on a friend's back and had it rubbed on him, I noticed a distinct swelling down there - a good 70% bigger and quite noticeable.