-Boston fern. I have one in my russians pen outside and they like it, and it looks awesome.
-Aloe- I found some on our property growing so I transplanted, idk how expensive but they look cool, and are edible.
-Hibsicus- if you do have the space, this is a good choice
-Pot/ Field Marigold
-I would get some lettuce/ kale/ safe flower seeds and plant them in the enclosure, block that part off until they grow big enough, then open that area up again. Or just plant those in pots and transplant when big enough.
Spider plants are another good one. But if you buy a plant from a nursery or anywhere really you should assume it was treated with chemical fertilizers unless you know otherwise. And that means you shouldn't let your tort nibble on it until it's been repotted in chemical free soil and then you should wait several months. Some people say up to a year... opinions differ on the matter.