Kirk, McCoy, and Spock around a campfire
A sing-along was a Human ritual around a campfire, frequently practiced after one consumed marshmallows.
Spock, Kirk, and McCoy were camping on shore leave in Yosemite National Park. Spock produced a dispenser of marshmallows, (calling them "marshmellons"), and told the others that after one consumed the treats, one must engage in a ritual known as a sing-along. Kirk and McCoy discussed several songs to sing including "Pack Up Your Troubles", "Moon over Rigel VII", and "Camptown Races" before settling on "Row, Row, Row Your Boat". (Star Trek V: The Final Frontier)
On two different occasions, Neelix suggested a singalong; while sharing accommodations with The Doctor (a successful ruse to get The Doctor to agree to turn the lights out so Neelix could sleep) (VOY: "Demon") and while looking after children when the USS Voyager's power was intentionally disrupted (a way to amuse the children in the dark). (VOY: "The Haunting of Deck Twelve")