As a lark is an idiomatic expression that means to do something just for fun or pleasure, without any particular reason or purpose. There are several synonyms that can be used instead of this expression. One of them is "for the heck of it", which conveys that someone is doing something just for fun or simply to try it out. Another synonym is "on a whim", which indicates that an action is taken spontaneously, without much thought or planning involved. "Just for kicks" is another similar expression that means doing something just to amuse oneself or for entertainment purposes. Overall, these synonyms convey the idea of doing something fun and enjoyable without any specific goal in mind.