Fun and Games is typically a part of the B Story, although it could be part of the A Story as well. But it's tone is undeniably lighthearted. It's the awkward first date of the romance novel; it's the game of pool at the bar after a long day on the case in a thriller novel (perhaps with a romantic interest, if it's part of the B Story). It's the arena of a unique sport in a fantasy novel.
The purpose of the Fun and Games, somewhat like the B Story, is to break up the potential tension in the story. But while the B Story can be used to insert a little lighthearted tone, the Fun and Games does add a bit of relief. It lets your character and your reader breathe within the story. It's not meant to distract from the intensity, but rather to ease it a bit for a period of time.
However, there can be a few concerns with the Fun and Games section. Primarily, it's really important to stay within the character's boundaries. Characters develop according to their qualities and circumstances, and no two characters develop exactly alike. Just as not all characters respond to conflict in the same way, not all characters experience fun in the same way. If your character is introverted, he or she is probably not going to party it up at a club in the Fun and Games section. Similarly, laughter-filled one-on-one coffee date probably isn't going to brighten the spirits of a sociable character.
But also, it's important not to get distracted as a writer. Fun and Games is meant to be a reprieve from the story, not a segue into a new story line. I've fallen into this trap before -- you get on a tangent and you just keep going. It's one reason I strongly advocate for outlining, rather than just sitting down to start writing. It's easy to get distracted from the path you're headed down, and diverting with contrasting scenes can derail your project completely if you're not careful.
What are some ways your character can "let loose" a little? Try not to be cliché here, but find authentic outlets that suit your character and your plot arc. How does this scene align with your overall story direction? How could it possibly get out of control?