carrots, apples, celery & lettuce (don't feed these 2 everyday bc it has a very high amount of fiber & don't give them iceberg lettuce bc it's very bad for their health), timothy hay (there are many different cuts of it and you want to make sure that it is not just a bunch of stems bc they are hard and sharp, making it more difficult to chew/digest and if it pokes your piggy in the eye it may lose its eye) there are also a variety of treats at the pet store that are for guinea pigs
They love fresh hay. They eat guinea pig food were you can find at pet stores. You can slice some cucumbers and feed it to them. Carrots, apples, and lettuce are good too. Make sure you don't give them too much food or you'll over fill them.