As all Guinea-Pig Slaves know, these little furries are very particular when it comes to their food.
Yes, they need their Vitamin C
Yes they have things they cannot eat
Yes they have things that they shouldn't eat alot of.
Mine use this information to be very judgey about what I give them. And sometimes I buy parsley from Supermarket S that they hoover up. But buy it from Supermarket M , they turn their noses up.
So.....are we sitting comfortably. We'll begin.
We have GP1, smooth boar 3.6yo
GP3, Rex boar 10 months
We did have GP2 (brother of the smooth ) Abby type (now RIP)
GP2 loved watermelon,grapes,strawberries,sweetcorn (especially with the leaves) beetroot,sprouts, leaves from the willow tree. He also had terrible gas 
vom , and if he had a day eating grass he had to have hay and pellets for supper , no veg .
GP1 loves grass, dandelions but not clover. Leaves from the pear tree.Parsley, he'd sell his mother for parsley. Carrots but only the ones with leaves, normal ones get left, leaf ones get devoured.
Watermelon, sweetcorn, spinach/rocket/watercress salad
Red /yellow/orange pepper but not green (so much that his pee went peach )
Banana
GP3 loves grass, yellow pepper, brocilli, dandelions, green beans,rocket, celery leaves but not the stalks.
Apple
GP1 has a liking for Cornetto cones
The aren't allowed coriander , it makes them narky.
They don't like mint, basil,
So, add to the list, the more the merrier (and I know there are pigs with eccentric tastes out there)