I am relatively new to guinea pigs, and I had the same question as you, but what I did was to buy some of everything fresh that I could find and give them as much as they could eat every day and throw out the stuff that they didn't eat. A couple of months later I would try the rejected food again, if they still didn't eat it I didn't buy it again. I read the advice here and other places on the Internet. I generally give them the same thing every day, but it's not everything in the list below as some things are seasonal.
Red will eat:
Parsley, carrots, celery leaves, cucumber peels (not the white part), sweet apples, green beans, and cauliflower sometimes. I pick grass, dandelion leaves, clover, hay (Timothy and orchard grass grow wild here), and mulberry leaves.
Sal will eat:
Parsley, carrots, celery sticks and leaves, green peppers, sugar snap peas, green beans, sweet apples, and beetroot. He also eats all the picked stuff that Red eats.
I've tried the big leafed greens such as kale and collards but neither will eat them, and I've tried other fruits but neither will eat them. Red tries all the new stuff once, and if he doesn't like it he doesn't go back.
Thanks, justagrrl, I didn't know they would eat cilantro, there's lots of it in this area.