The main problem I find is the constant balancing act. The thing with piggies is, because they're fairly smart, they get bored quite quickly and even their favourite foods will get left if you feed it to them too often.
We try to rotate things we know we can trust and still maintain a balance. It helps to draw up a chart of about two or three weeks that you can follow to make sure that your piggies are still interested in their fresh veg and you've got a good balance of vitamin C, fibre and the right levels of protein.
There are a few lettuces you can use to base things on (romain leaf, little gem but not iceberg or cos - too high in phosphates and water). Add to this some other leaves: cauliflower leaves (once a week or a little less often even);flat-leaf parsley (the same); curly kale (this has quite a bit of calcium so beware, feed maybe twice a month), basically try to keep the water, vitamin A, phosphate and calcium low but vitamin C and fibre high. Grass is a really good and safe way to feed fresh leaves and it's free
Now even romian lettuce is high in vitamin A, but it has almost equal levels of vitamin C so you can use it once a week but then you have to make sure that the rest of the fresh veg is extremely low in vitamin A for the rest of the week and so on...
Then for veggies themselves, try to replace carrot with the occasional piece of cucumber, pepper, banana skin (with a tiny bit of banana as a rare treat), watermelon rind (again, with a small amount of flesh as a treat), the occasional strawberry (once a month or less) or other similar fruit.
The important thing is to make sure that fresh veg/fruit etc. accounts for only 10% or so of your piggies' diet. then you can give them either 90% hay or 80% hay and 10% dried food. Dried food really is only if you aren't sure your piggies are getting everything they need from fresh food and hay.
Anyway, sorry for rambling on about stuff you probably already know - hope some of this is helpful
