Luckily most canary food manufacturers know how fickle the diet of the canary can be and have developed proper mixtures aimed at keeping your bird's health in check.
Canary grass seed should be a staple of your canary's diet and you should always check the ingredients to make sure this ingredient is at least 50% of the mixture if you're buying a seed mixture (this will vary for special "moulting mixes" or "color feed).
Canary grass seed originates from the same place canaries originally do (Mediterranean), and while it doesn't give them all the nutrition they need: It give them more than other foods. They also require a food that's high in various vitamins and minerals (calcium is one of the important minerals in any good canary food).
One of the great things about owning a canary is their ability to entertain and look gorgeous, without needing your constant attention. Since the canary is known as a "birds bird", rather than a "people bird".
You can feed your canary almost any fruit, nut, or vegetable that you would eat (other than avocado which contains too much fat).