The phrase "on the side" is commonly used to refer to secondary or additional activities, jobs, or interests that are pursued alongside main ones. There are several synonyms for this phrase, such as "as a sideline," "incidentally," "in addition," "extra," "supplementary," "auxiliary," "subsidiary," "secondary," and "parallel." These words can be used interchangeably to add nuance and depth to one's writing or speech. For example, instead of saying "I have a job on the side," one could say "I work as a sideline to support my main income" or "I do additional work on the side to make ends meet".