Vitamin drops don't really work, I've heard this a lot so I ran it by my vet she said the vit c is soon weakened in the water making it pointless really. The best way to get vit c into your piggies is through veg (I've heard not to give carrots every day too) the link that lisaali shared should help you, try different things from the list and you can try the ones they don't like again at a later date they may start liking them. My boys have hay all day I always keep it topped up, a large bowl of veg in the morning, grass during their run time if the weather is nice, a little treat in the afternoon (which changes daily) such as a bit more veg or dandelion/raspberry leaves with a few slices of apple or some grapes, Then pellets in the evening and fresh water twice a day. May be use the guild to find things you eat eg things you have in the fridge and try them on them

were having a meal tomorrow that has spinach, carrots, fresh herbs etc in it so my boys get some of them before I add the rest to my cooking same with our Sunday meal they have parsnip, green beans, cabbage, carrot, fresh mini corn, broccoli , colly etc (not that we have that all in one meal lol)
Hope that was of some help.
Kell x