Most guinea pigs aren’t known for overeating, but it’s still important to make sure they get a balanced diet. About 1/8 cup a day of pellets is usually enough, with some fresh vegetables and an unlimited amount of hay. A cup of veggies per day is sufficient.
These should be leafy greens like spinach, romaine lettuce, or kale. You might add some carrots and zucchini once or twice a week for variety. The hay keeps their digestive systems healthy and minimizes the growth of their teeth.