At the moment, I have to say regular sex, since we have a couple of cockerels too many

.
However, when we kept ours in a run, I once made a wee device out of an old plastic tennis ball thing - I just drilled one small hole and one big hole in it, and then filled it with grain through the big hole, and sealed it up with duct tape. Every time it rolled, some grain would fall out of the small hole. They used to chase it around for hours!