In the wild, the cavy's diet consisted of dried leaves, dried grass, and other dried vegetation with little nutrition value. Though our domesticated cavies are not the same as their wild cousins, their systems are still created to handle mainly hay and dry pellets. Feeding too many veggies can cause your cavy to become picky and can give your cavy diarrhea. Too many fruits will make your cavy obese and can also cause stomach problems. Of course they like the fresh veggies and fruits better - but many people like sweets better than healthy foods, too. Practice moderation when giving your cavy fresh fruits and vegetables - especially when giving fruit, which is very high in sugars that the cavy is not made to handle much of. This is not to say you shouldn't feed your cavy fresh food, just do so in moderation. A good cavy diet is as follows:

70% hay, 20% pellets, 10% treats (veggies/fruit)

If you feed your cavy a variety of safe treats when they are young, they will be more willing to try new foods when they are older.

Veggies

Do not feed your cavy spicy or pickled vegetables.

Veggies marked with an * should only be given occasionally and not as a regular treat because they can, over time, give your cavy stones

Fruit

Fruits marked with an * have been known to cause mouth sores when fed too often

Some treats to avoid

- Fruit/nut/honey bars (too much sugar, and seeds are not a normal cavy food)

- dehydrated/frozen foods

- yogurt drops (too high in sugar)

Most pet stores sell a variety of treats for cavies, but most of them are processed and contain sugar and ingredients that are not a normal part of a cavy's diet. While a cavy might eat them when offered, that doesn't mean they're good for them.  Pet stores sell them because people buy them. Help your wallet and your piggy, and stick to natural stuff - you can't beat fresh hay, veggies, and occasional fruits.