I have trees & shrubs ~ which they love ~ so don't grub them out if you have them; build around them. Chickens are rough on plants so you'll have a hard time establishing anything inside the run unless it's already really well established. I prefer my run doesn't look like the local junk yard so I'm adding stags, elks, & air plants that they won't be able to reach & decimate. Piles of anything: weeds, grass clippings, leaves, sand...I've even used sea grass. Perches. Old stumps.