I can recall three-ish scenes I would classify as lighthearted in XIII. The Hope and Vanille chat on Pulse in Chapter 11. Vanille messing with the sheep. And um... Maybe Sazh and the giant cactuar. I guess there could be more, but those scenes are so few and far between and now that I think about it, all happen on Pulse.
I'm the rare breed around here who actually likes Vanille as a character, despite her awkward moans. lol
Also most of Nautilus. And Sazh lagging behind Lightning in the beginning of the game. Then Sazh's baby chocobo.