This time, they were joined by two special guests -- Keegan-Michael Key as a counselor and Billy Crystal as a fellow camper.
The clip begins with Timberlake telling Fallon he's wide awake as a result of eating "like, seven Charleston Chews." The two then play a game of Two Truths and a Lie, later bonding over their mutual love of "What's Up" by 4 Non Blondes… and waking up the other campers.
That's when they receive a visit from Key, shortly before Crystal stops by.
"You were just singing a song?" Key asks. "Is that what you were doing? Because it sounded like you were just drowning in a lake."
See what happens next by watching the full 10-minute segment below: