I put small boxes in - they love jumping on them, and running in & out of them.
I also have a dog crate panel covering my brooder - I hang a piece of string down into the brooder, just 1-2" above their heads, but make sure to tie it firmly. They love jumping up at the string, and I have some adults that now will jump up to take a treat from my hand.
I've also put in a pice of small wood...like a small log to burn in your fireplace. But, make sure there aren't any loose bark pieces on it, first. They quickly learn how to "sharpen" their beaks on it, and they love jumping on & off of it.