Safe treats (in moderation)
Treats should only be fed to chinchillas over 6 months, be fully free from pesticides and dried:

Alfalfa Leaf
Picture
Apple Blossom
Apple twigs
Basil
Bee Pollen
Blessed Thistle
Cactus Flower
Calendula
Carnations
Chamomile
Clover Blossom (red)
Cornflower
Dandelion Leaf/root
Echinacea
Picture
Goji Berries very limited
Hawthorn Leaf/Berries
Hibiscus
Jasmine
Lavender
Lemon Balm
​Linden Leaf
Mallow
​Marigold
Marshmallow Root
Milk Thistle
Mint
Nasturtiums
Nettle Leaf
steel cut only Oats
Oat Straw
Oregano
Peony
Peppermint
white willow twigs and sticks

If in any doubt, get a simple food dehydrator and run everything through one of those first! Chins get most of their moisture from water or the hay itself, and you never want treats or other foods to have any excess moisture, as it can cause fermentation which can be life threatening to a chinchilla's slow digestive system.