When I was younger, probably about 16, during the summer we had an old hammock hanging up in the backyard, and when you laid in it it hung pretty close to the ground.
One of my favorite things to do, I would wait until the grass was kind of tall, go outside when I was home alone and lay naked in the hammock. When I was fully hard, I would roll over and poke my penis through one of the holes so it was sticking straight down. The grass was just tall enough, and my penis was just long enough for the head to brush the top of the grass. I would then rock back and forth, letting the grass scrape and poke at the head. It felt so good, I had to squeeze the hammock with my fingers and toes so I wouldn't roll off. When I came, it was much stronger than with my hand, and I didn't have any mess to clean up.
I did this off and on for a whole summer, until one day my mom came home early and saw what I was doing. I knew I was in for it, and as soon as I walked inside she started yelling at me, asking what the neighbors would think if they saw me naked and masturbating in the yard and to keep that in my bedroom. All I could do was stand there and apologize, still naked and still hard. Eventually she told me to go get dressed, and after I went up to my room, I quickly stroked myself off into a dirty sock. I still thought about being in the hammock, but that was the last day I actually did it.