Mine have weeds, vegetables and fruit.
They do need a constant supply of hay and I use Timothy hay.
With regard to commercial guinea pig food....I use Burgess Excell which I believe to be the best on the market. The ones like 'Gerty' which have a variety of things in is ok but the piggies will pick out what they like and leave the rest. The complete pellets may look dull and boring but they do ensure that the piggies get all the nutrients.
There are lots of caresheets on the web that will give you details of suitable fresh foods or you could look on some guinea pig forums...most have sections on feeding.
They are super pets, mine are very precious. Good luck!
