Feed a floret or chunk of stem per piggy 2-3 times a week.
The rule of thumb is most veg and sprigs of herbs are best fed 2-3 times a week. Fruit/tomato should not be fed more than twice weekly to avoid mouth and lip sores.
High calcium veg like spinach, kale and Asian cabbages should only be an occasional treat. I feed a slice of sweet pepper/capsicum, a slice of cucumber, a chunk of celery daily.
You can also feed a sprig of coriander/cilantro daily.
Cabbages and crucifera like broccoli or cauliflower (including leaves and stems) are fine to feed if they are introduced slowly and always fed as part of a mix; but you should take them off the menu for any piggy with digestive issues. They are medium high in calcium, but a good source of vitamin C.
As long as you do not feed anything too often or in large quantities, you won't go badly wrong.

Do not introduce more than one new veg or herb at once with any piggy, and preferably always as part of a mix in order to prevent diarrhea or bloating.