One of the best thing my guinea pig likes is: Apples, Carrots and grass. You can also try to push in a kong ( not very tight, and always supervise guinea pig, their head can easily get stuck) and grass is always a fav for my guinea pigs

Thanks for telling me, my guinea pig has been getting fat lately so i gave him more fruits, well...now i know that some are fatteningPlease be aware that too much apples (or any fruit incl. tomato) can cause cheilitis (lip infection) and that too much carrot is fattening.
Guinea pigs are individualists; they have all got their own favourites.