Guinea pig superfood is unlimited hay and fibre. Guinea pigs have evolved on a nutritionally poor diet of mainly grass and dry grass (hay) with only occasional perks of other stuff. That is how their digestive system is laid out for and how will function best.
They cannot make their own vitamin C, so it is good to either include a slice of sweet pepper of any colour or a sprig of fresh herb into their daily diet, but for general optimal health, you won't do them any long term favours by overdoing things. A general balanced fibre based diet with only a small amount of fresh veg is best for good health and longevity. https://www.theguineapigforum.co.uk...or-a-balanced-general-guinea-pig-diet.116460/