Think about your audience. Some of the best stories that incorporate levity were originally crafted with the intention of pleasing very particular people, often children (Alice in Wonderland, Winnie the Pooh). One of the reasons MacDonald claimed to write ‘for the childlike’, I think, is because children are more willing to engage dialogically and spontaneously with stories. Maybe try writing a story for a cousin, niece, godson or other child. In addition to forcing you to think outside of yourself, it will also make you think carefully about each word and sentence and give you an impetus for brevity.