Sunblock
I love having someone apply sunblock. Sort of combines a necessary function of being on the beach with the sensual feelings of a good massage. When done slowly and skillfully, a hard on usually results. From there it depends on the location. Don't mind having an audience either!
Last time I traded sunblock application with a stranger on the beach we definitely were slightly more liberal than necessary, but not more than that as it was a social beach and not "that kind" of beach. Otherwise I'm sure we both would have gotten it everywhere.
Years ago, I went to a beach that had a clothing Optional side. In one section most of the gay men would go and hang nude. There were lots of bushes in the area for guys to go and play. I was there to hand nude, a guy asked if I want Sunblock on and I did not see any harm in it. Well, the guy started on my back and worked his hands to my butt and grabbed my cock. I let him continue , the more he rubbed the harder I got. I loved when his fingers rubbed my crack and he would finger me and grab me and rubbed . We went into the bushes and got each other off. I even let him try to put his monster in me. I was too tight for him. Yet we still had fun, at least I did not get sunburned.