I used to wear a large silicone ball collar with my gf and she said she loved the feel of them banging against her clit when we had sex doggy style. She also said that when she reached back to play with her clit she could grab my swinging sack and squeeze them hard, sometimes a bit too hard. We used to walking naked out on deserted moorland quite a lot and towards the end of the relationship she asked if she could tie my balls to a heavy rock, when I say heavy I mean about a pound maybe more depended on the rock or brick she found, I said yes so I was walking around with this collar on and a rock dangling from my balls. Despite the sound of it the feeling was quite nice, although we did get caught a couple of times and it's not so easy trying to explain the rock pendulating between my legs