Just make sure to give them a tiny piece if you want to give a treat.
Once a week or a teeny tiny piece once a day, is okay.
If you give too much of treats or too often, you can have issues with their tummies like diarrhea or constipation among other things. I wouldn't recommend citrus fruits or melons. Treats that people have given that are okay in moderation, are raisins, dried apple or dried bananas as mentioned before.
Break or cut it up into smaller pieces. While our tummies do fine with treats, our tummies are also much bigger than a chinchillas. So what may seem a tiny amount of sugar in a treat to us and our digestive tracts, can be too much for them with their little digestive tracts. It is why moderation is so important when it comes to treats.