Some of the things that symbolize good (aside from the good witches) are love, humility and wisdom, light, dance, Meg's name: these all add up to the human heart and brain working together, which is, of course, the antithesis of (the opposite of) the symbol of pure evil represented by the bodiless brain that tyrannizes Camazotz.
As illustration, Meg's name means "pearl" and symbolizes luster in living, which is a symbol of good. Dance represents harmony even though there is variety in the unity of movement, which is a symbol of good.
[They were] above the crystal river that flowed through the garden, flying in what Meg was sure must be a kind of dance, moving in and out above the trees.
Humility and wisdom, which are symbols of good, oppose tyranny and lead to solutions. To illustrate, Charles has intelligence without humility or wisdom and so winds up a servant to evil. Other symbols of good are crystal water, gardens, undersea plants, and stars.