Now, the answer to this is cheating somewhat, because I have technically tried it.
Mud. I want to play in the mud. That's the short version. The long version will be behind the cut.
When I was really young... probably somewhere in the 6-8 years old range, I remember watching a childrens show on TV, the Great Space Coaster. I remember very little about this show... except for this one vivid memory of a Gary Gnu skit.
Gary Gnu was a puppet Gnu behind a news desk. The theme was, "No gnews is good gnews, so here's the gnews, with Gary Gnu." This particular skit showed a group of guys in a really muddy field playing football. Then the camera would pan left, and we would see a group of girls, watching the boys get all dirty while giggling. Pan right, and we see a closer look at the boys, pan left again, and we zoom closer on the girl in front.
The girl is wearing an amazingly tight white t-shirt... the very example of form-fitting. The whiteness of this shirt was amazing - clean, virginal, pure. You could clearly see the form of her developing breasts.
Pan right again, and there is a boy holding up a pile of mud, getting ready to throw.
Pan back left, and the mud lands on the girl, perfectally covering her right breast.
I must have masturbated to that image millions of times over my developing years. I grew obsessed with the concept, and would look furiously for photos of mud wrestling anywhere I could find it. This is my only memory of the show, and I can replay it perfectally in my head - from Gary Gnu's opening speech, to the throw. My mind slows it down to a frame by frame affair. Vivid.
One year at Leather Retreat, a close friend and I attempted the scene. On a rainy day, we went out in front of my cabin, got naked, and rolled around together in a mud puddle. It didn't work - it was way too cold to be naked out, it was still raining cold rain, and the mud smelled AWFUL. I soon learned that biological mud is no good for this. (I will say the piss play that we attached to the scene was wonderful.)
Of course, we never did plan out how we were going to clean ourselves off, and wound up taking a "walk of shame" naked and dirty up the path in front of lots of cabins to a barn that had a water hose connected to it. Of course the water coming out was freezing.
I rule that attempt as a false start, and thus I still have not fullfilled this fantasy of mine.
So, that's what I want to try. Two or more people, naked, covered in mud, writhing all around together.