I have to say, weird as it may seem, my chickens hate cabbage! They hate all cruciferous vegetables so that whole cabbage tether ball thing is not an option for us.
What they do love, though, are peanuts. I will throw a section of straw down for them and then throw a cup of peanuts into it where they will have fun for hours scratching through it looking for goodies. I just let the straw get nasty over a couple days and then toss it in the compost pile. I make sure, also, that they get plenty of out of the run time to play in the yard which makes it so that when they are confined, they are not all that miserable.
I have to chime in on one last thing, though, and that is the heat in your coop. I feel as though 50 is too warm for all winter. We live on the Atlantic coast where we get wind, rain, sleet, and freezing rain and I have never heated there coop. I understand from a human perspective we like to be toasty warm, but your birds are actually naturally equipped to withstand cold cold temperatures. If they are going to be outside birds forever than they had better beef it up to be outside birds even when that means temps in the 20s or lower. You can mess with there molting and natural ability to withstand cold temps by keeping them heated.
All of this is just my opinion from my own experience, and you don't have to listen, but I think I have some of the happiest chickeroos this side of the Mississippi.
Happy chickening!