Lots of options open to you, but you must make sure that the plants not poisonous to chickens, and protect the base of the plants from them so they don't dig them up or damage the roots.
You could try an evergreen shrub like viburnum tinus - winter flowering and bees love it, or small fruit trees. Grass may be a little more tricky as they'll probably dig through it pretty quickly (depending on size of enclosure). You could also try a climber like clematis Montana if the area is fenced off, this flowers in the spring and will cover a fence panel in a couple of years easily. If you've got net over the top though, climbers will be heavy, so annual clipping back would be needed.
You could also try a small silver birch tree if the enclosure is big enough, provides very pretty light shade, small birds love it and the leaves are antiseptic, so good to add to compost bins and for the soil.