The Light Princess is much shorter than The Day Boy and the Night Girl, and more whimsical in tone than it or the other two stories it's anthologized with. MacDonald takes up the ironic tone common to Victorian fairy-tale parodists of the time, eyes rolling as he tells you that of course everybody knew about fairy-tale rules but this sort of thing happens anyway.
This princess is cursed by a jilted aunt, also referred to as a princess (reminding you that princess status does not equate to goodness) to be unaffected by gravity. MacDonald's science fiction credentials are shown here, as well, as he works through the consequences of this anomaly. The princess is weightless, and does all the fun things we see astronauts doing in space, but not only is her body disconnected from all weight, in good MacDonald fashion so is her heart.
The princess can't take anything seriously. She's never sad, but she's never truly happy - her laughter lacks inertia. This is shown as a terrible disability, much worse than simple weightlessness, and there is a delightful cameo from a pair of Chinese philosophers attempting to help her father fix this. The princess is not massless, though, as she finds by accident that while submerged in the lake by the castle her weight returns, and she is able to think heavy thoughts.
Then she falls in love with a prince, and her wicked aunt gets even more wicked, and we reach a denoument that's all the heavier for the flighty, drifting introduction. Highly recommended.
This story was my introduction to MacDonald and seems as good a place as any to start with him.