Image via WikipediaGuinea pigs like a variety of fresh fruits and veggies and should eat foods high in vitamin C here is a short list they will like

Apples- no seeds
Bananas- in moderation
Carrots- in moderation
Cucumbers
Dandelions
Grapes
Green beans
Leafy greens like kale or parsley
Oranges
Strawberries

Cavies should have timothy hay or timothy hay pellets available to them throughout the day since they eat all around the clock. Your cruciferous vegetables like broccoli, cabbage, or brussel sprouts will cause gas in guinea pigs like in humans so give sparingly.

Do not give your guinea pig potatoes, rhubarb, onions, tomato leaves or stalks, or garlic these are poisonous to your guinea pig, while celery is not poisonous it has strings and your guinea pig can choke on those.

Give your guinea pig one cup of fruit and/or veggies each day you can feed them a portion in the morning and evening if you choose.