I am by no means an expert in avoiding marks but in my experience I’ve never had marks or bruising from a moderate amount of ball busting using a fist or a paddle. You can likely get away with kicking but i have had some cuts from the impact of the tip of a high heel shoe. I would say the pain is intense during the session but completely disappears within a couple of hours.

I would stay away from whips or canes to that area. Both of which have caused welts and bruises for me in the past. Id also be careful of clothespin or clamps

For NT i dont do as much. But pinching with fingers, clothespins, or clamps have never caused mark for me. But do cause pain (similar to chaffing after running a marathon) that lasts a couple of days