In addition to hay and pelleted feed, provide your guinea pig with some fresh raw vegetables daily. I fill two ceramic custard cups for my four girls, and throw in about 3 or 4 leaves of lettuce. Don't feed too much fresh veggies, because it will cause diarrhea. Make sure all fresh vegetables are washed and free of pesticide residue. Some guinea pigs are sensitive to the acid in applies, and their skin will react.
Dandelion greens are well-loved by guinea pigs.
Wash them before feeding them to your guinea pig. In warm temperatures, take your guinea pigs outside for exercise. Make sure the grass is free of chemicals and waste. Water must be constantly available to your guinea pig, period. Get a sipper bottle with an aluminum tube, since guinea pigs nibble the end.