For me it is pullups. My first orgasm was while climbing rope during our break period in JHS, I did not know what it was, I just knew that higher I climbed the stronger this sensation in my crotch got. Finally I came and I held the rope tightly as it subsided. Lucky for me I didn't start shooting for another three or four months.
I used that rope regluarly until I found the pullup bar would have the same effect and it was private.
I actually used to do pullups in the school bathrooms all the way until my sophmore year of college. I used to be able to cum just from that. Then I got a permanent gf. End of story.