My chickens are very curious, so I like to change it up on them. I will rearrange the perches, or put a stump in, or a cinder block. I put the hay down in big chunks and let the chickens distribute it. Last fall I gathered leaves and pine needles, dumped piles for them to scratch through. Right now is freezing, so have deep hay in covered pen, if I "scratch" down to the dirt in a couple places the chickens hop into the "hole" and start scratching away looking for a fallen morsel. And then of course are the treats.
On another post, a fellow BackYard member posted she gives her hens boxes and they love them. She leaves the boxes in for a few days and then switches them out. Do a search on boxes for the specific post - it sounds like a great idea that I am going to try.