There is no such thing as chinchilla safe fruit and veggies, unless maybe you are talking rosehips as a "fruit". Fruits, like the banana you mentioned, are way too high in sugars which can lead to digestive upset, liver and kidney failure, diabetes, seizures, and tooth decay to name a few. Veggies also have natural sugars and can cause gas and bloat, since chins can't pass gas easily it can be fatal. Just because your chin hasn't dropped dead doesn't mean it's safe, some of the issues like the organ failure and diabetes can take years to develop, think of it like smoking a cigarette, one is unlikely to kill you but each one is doing damage. I posted a list of safe treats on this thread last year
Treats
NigelAndPeanut, one other thing, be very careful with using cardboard, only use it when you can supervise. Most chins just shred it, but some do chew and swallow it which is not safe. If they swallow cardboard it can cause a gut blockage and kill the chin. Also be careful with using the glue as well, although I'm pretty sure hot glue is non toxic it's not digestible, if the chin swallows a glob of it it can also cause a blockage.