I live in zone 8b so not all the things I can grow you can probably grow but I do experiment a ton with different plants. If you buy a plant and the peafowl start eating on it you can always dig it up and put it outside of the pen for decoration that way you save your plant.
Some lantanas grow all over the pen naturally. Under one of my biggest lantanas my peahen Damsel made a nest. It was a good hidden place and very shady so I hope it will be a future nest site for others. Lantanas attract lots of butterflies and some bees so you get that added beauty. Lantanas are said to have poisonous berries, but my birds never eat anything off of the plant they leave them alone so I have never had any problems with it. I personally love this plant. It dies back in the winter but comes back very quickly when it warms up.
Here is Damsel nesting under the lantana.
I also have 2 different kinds of clumping bamboo in the peafowl pen. Make sure you get CLUMPING bamboo. Clumping bamboo keeps shoots in a circular area instead of sending runners out everywhere causing your pen to be covered in bamboo. I have this green bamboo (it has 8 new shoots this year) and I have a smaller clump of this golden bamboo with green stripes. This is another thing the peafowl will not eat and it looks very very pretty in the pen the more mature it gets and it provides great shade. You can definitely have bamboo in zone 7a!
I also let the grass grow in the pen. We don't cut the grass we just leave it for the peafowl to peck at. This is bahiagrass, in your lawn it is anoying because it grows really fast and it is a weed, but in the pen the peafowl slide their beaks down the V shaped top and get all the seeds off of it.
Here is a picture from early Spring of this year of part of the pen. The grass wasn't tall yet. The plant in the bottom left corner is a Japanese maple. Japanese maples are sooo beautiful! Mine has purple-green leaves. The leaves change colors. In the spring the leaves were mostly green and I think as it gets colder they turn purple. The plant in the center is a fatsia, I love that plant a ton because it looks tropical and has awesome leaves and it stays green all year round.
I have some native trees in the pen too, mainly a few young oak trees. Then I have a wild pear or apple tree, one of the two. When the leaves fall the peafowl like to eat them and sometimes they jump up trying to reach new leaves but they don't destroy the trees.
Cast iron plants are really nice looking too. The peafowl never eat them. When I first planted them they pecked at them a little, but they never showed an interest in eating them.
As for grasses I tried some purple fountain grass and once winter came it died and never came back. It didn't work well for me it never really grew and so I just took it out. The peafowl never ate it though. The grass I use now is zebra grass. I think it has some other names. Any grass like zebra grass (like giant miscanthus) that gets really tall and fountainy I like, I just don't know why my zebra grass hasn't gotten that tall. I have seen pictures of other peafowl breeders that have beautiful clumps of zebra grass around their pens, I am so jealous. Oh and I did try a small pampas grass plant that I got at Walmart but the peafowl scratched it up. Some plants they don't eat but they scratch all the leaves off.
One of my newest additions is a Trachycarpus fortunei (type of palm that can be grown in zone 7). I want to add a giant rhubarb. I tried to have bananas in the pen but bananas are such soft plants the peafowl just tore it up. I also want to try a hardy Hibiscus, some Tetrapanax papyrifer, I planted a Mahonia that I dug up but I think it is dead because I dug it up wrong but the peas didn't eat it, I have heavenly bamboo in the pen, the peafowl don't eat it but they will eat some new leaves so try to get a tall one (it isn't a bamboo), I want to add roses to the pen, and lots of other things I want to plant. Oh and I also have crepe myrtle in the pen which we never cut back because I want it to be nice and tall. It has pretty hot pink blooms right now. I really like landscaping the pen.