Things chickens CANT have????

 know the dangers of over feeding the chooks, and that my ex-batts are only going to need their pellets or crumb, but I'm going to HAVE to spoil them occasionally.
I have loads of sunflowers growing in my garden, if I dried the heads and saved them for the winter, would my girlies be able to have them, hung up, as a treat and something to do?? And is there anything else that is a propper no-no for my ladies, understandable, meat isnt good and I know not to over feed but in the deep dark winter, I'm going to have to give them something nice once in a while 

Melons they eat it all,
also they like any cake thet is left in the tin but i add water to it so that it does'nt swell up inside them