"In fun" is a common phrase used to indicate a lighthearted or playful tone. When looking for synonyms to convey this tone, there are several options. "In jest" is a comparable phrase meaning something is done in a playful or humorous way. Alternatively, "tongue-in-cheek" is another phrase used to convey a sense of whimsy or sarcasm. "Jocularly" is a word that can be used to describe a situation or comment that is made in a joking manner. Lastly, "amusingly" can be used to describe something that is entertaining or humorous in a playful way. Regardless of which synonym is chosen, each can be effectively used to indicate a playful tone.