Your canary loves variety, so plan to give him treats in addition to his staple seed or pellet diet.  Keep in mind that treats should be given in small portions.  A treat is supposed to be a "treat", and should never replace their basic diet. 

Almost any vegetable will please him, especially broccoli, kale and other leafy greens.  Pieces of cucumber, corn kernels, and peas will also intrigue him.  Fruits such as grapes cut in half, pieces of apple, or berries will give him added nutrition.  Leftover pieces of bread, cornbread, or crackers will also be eaten with enthusiasm, but these are high in carbohydrates, so watch the portions.  Be especially careful to avoid giving them foods with sugars added.

If your yard grows dandelions, pull one up roots and all, wash thoroughly, and let them enjoy it.  Be certain that no pesticides or herbicides were used on the yard it came from. 

It is important to wash all vegetables or fruits before giving them to your bird. 

Just as excess treats will make humans pudgy, be cautious about giving too many treats to your canary.  Your birds will eat nearly everything you put in front of them, and can get obese and unhealthy. 

Monitor your bird's weight.  The way to check to see if they are getting too fat is to catch them and feel along their breast bone.  If the breast bone in indented and forms a groove between either side of their chest, the bird is getting too plump.  Put them on a reduced calorie diet...cutting back on the treats until their weight is normal.  Their breast bone should be even with the sides of their chest.

If the breast bone is a sharp ridge that rises above their chest, your bird is underweight, which is a dangerous condition.  Underweight birds are more susceptible to illness, so you need to increase caloric intake, offering higher carbohydrates such as crackers and breads, and more oily seeds such as flax.