You can offer them dandelion greens, collards, kale, endive, escarole, mustard greens, carrot, squash, sweet potato, sweet red pepper, zucchini and once in a while apple, berries, pear, melon, etc. For the leafy greens just remove any thick stiff stems. Cut the veggies and fruit into appropriate sized pieces or slivers.
Any live plants should be well-washed and non-toxic since they will likely eat them too. I don't recommend using fake plants for them because they might nibble them too.