Here's a rundown of guinea pig food...

• Good foods for guinea pigs to eat are: apples, bananas, pears, strawberries, grapefruit, honeydew melon, kiwis, tangerines, yellow plums, oranges, watermelon, grapes, lemons, cauliflower leaves, broccoli, turnips, peppers, parsley, red beets, cucumbers, celery, spinach, tomatoes, and zucchini.
•  Bad foods for guinea pigs to eat are: Iceberg lettuce, all varieties of green cabbage, chinese cabbage, cauliflower, and red cabbage.
•  Poisonous foods for guinea pigs to eat are: Potato peelings and raw beans. As for the actual potato, there has been some confusion about whether they are or aren't poisonous to guinea pigs. To be on the safe side, I would suggest not feeding your guinea pig potatoes. For a more comlete list of plants that are poisonous to guinea pigs, visit the Poisonous Foods page.
•  Hay is another good food for guinea pigs. It is the basic food for guinea pigs. If you feed them hay along with water, it is suitable for the winter! This is only if no fresh veggies or fruits are available.
•  You can gather plants from outside to give to guinea pigs. They absolutely love outdoor greens! Just make sure that there are no flowers or dried tree leaves in the bundle you pick. If there are any leaves or flowers in it, cut them out and give only the plant leaves and stalks to your guinea pig. Grass is also considered a treat to guinea pigs! If you do decide to collect outdoor plants, do not get them if they grow near roads or sidewalks, or have been treated with pesticides!
• Toilet paper tubes and paper towel tubes are completely fine for guinea pigs to eat.  It gives them something to chew on to help their teeth, as well as something to play with. It is also like an in-between meal for them, when they aren’t eating anything else.
•  Pellets are a part of a regular guinea pig diet. They have some vitamins and minerals added, so they are really healthy. Guinea pigs don’t consider them a treat unless there are extra dried fruits or nuts added to them.
•  Salt and mineral wheels are optional. Salt wheels are exactly what they are called. It is salt formed into the shape of a wheel that can be attached to the cage. Then the guinea pig can chew on it anytime it feels like it! The mineral wheel is a mix of minerals and salt. It is usually a reddish color, unlike the salt wheel that is always white.
•  Vitamin C is very important to guinea pigs! They can get this from fresh fruits. Oranges have a particular big amount of Vitamin C. Vitamin C helps fight infections in guinea pigs, as well as to heal wounds. It also helps break down and metabolize proteins in the guinea pig's diet.
•  Clean and fresh water is necessary for guinea pigs to live. It is pretty much necessary for any living thing on Earth! Change the water whenever needed, preferably once every two-three days.
•  The entire digestive tract of a guinea pig is about 2.3 meters long.
•  These materials are needed for a guinea pigs' survival: Vitamins, minerals, carbohydrate, fats, proteins, and of course, water!
•  These vitamins are needed for a guinea pig to survive: Vitamin A, Vitamin B12, Thiamine, Riboflavin, Vitamin D, Vitamin E, and Vitamin K.
•  These minerals are needed for a guinea pig to survive: Calcium, Copper, Iron, Magnesium, Phosphorous, Potassium, and Sodium.
•  Guinea pigs are not able to store water for a long period of time, and that is why it is necessary to replenish their water any time the water bottle gets low. Every cell needs the water, so guinea pigs have to drink almost constantly.
•  Guinea pigs are also able to eat dirt. This may sound funny, but it is true. Dirt has vitamins and minerals and nutrients that guinea pigs need, and apparently, they seem to like it!
• Guinea pigs will occasionally eat their own feces (poops). The feces still contain some leftover nutrients, and so the guinea pigs eat them. This is sort of a way of recycling. It will not hurt them, and it will actually help them live, because of the materials within them.