Pet stores usually sell guinea pig treats. Although your pets will probably love them, you need to make sure there's not too much sugar and chemical additives in the treat. The treats often contain nuts or honey, which are fine as a very occasional nibble, but can cause obesity and tooth problems if fed in excess.
Many guinea pigs will be just as happy with less common fresh foods as a treat such as spinach, or a slice of apple. All these things should only be fed occasionally.
If feeding treats, make sure your guinea pigs burn off the extra load with plenty of exercise
One way of ensuring your furry friends burn off the extra calories they may acquire through eating the less healthy stuff is to hide the treats in the enclosure. Most GPs will enjoy hunting for them. This can be done using specially made guinea pig treat toys - something they can have fun trying to gnaw or break their way into, to reach the yummy treat inside. This way, you provide stimulation, exercise, and a tasty reward too.