Guinea pigs should have a variety of foods to choose from- guinea pig commercial food pellets, cereal, toast, fruit, vegetables and greens. Rather than restricting them to one kind of food, let them choose. Animals possess what is called "nutritional wisdom", but they can't use it if they have limited choice.
These next foods are what I like to give my guinea pigs and are not based on a professional opinion.
Carrots- these are my guinea pigs favourite food they will jump, run or flip just to get it from my hand.
Alfalfa- this is also a favourite of my g-pigs, from what I know it is good for them and they absolutely adore it.
Apples- my g-pigs will usually only eat a little bit of this but never finish it.
Celery- mine don't mind this food just try to peel off some of the loose strings that are on them.
If there are any other foods you would like to know are safe or not e-mail me.