There's a comprehensive feeding guide somewhere at the top of the "food" section of the forum, I cant link it from my phone but hopefully someone else can if you can't find it! Peppers (bell peppers) and coriander/cilantro are good daily or at least regular veggies high in the vit C that piggies need- can't go wrong with a slice of pepper and 3 or 4 stems of coriander. Cucumber is also a popular daily veg, a slice each. Baby leaf salad mix is good, maybe a handful, or little gem or romaine lettuce, a small leaf or half a big leaf each. Spinach, kale, parsley or dill are good foods but high calcium so fed sparingly once or twice a week. Occasional carrots or babycorn but not too much- these are very high calorie- just a slice, once or twice a week as a treat, definitely not a whole one each! And my piggies loooove beansprouts which are a good source of vit C but not all piggies like them...
Hope this helps!