I teach high school so I can't assume the same rules apply, but this is a great question!

For me, it's all about parameters and routines. I used to think in my first years of teaching it was about finding a literal balance but really it's more about shifting between super fun and super serious in a snippet. And training your class to do it.

I generally start the year more seriously but hint at the fun energy, then I gradually "try it out" but constantly bring the class back and keep shifting between the two. Leaving 5 minutes at the end of the period is often a great strategy too... when their work is done, walk around and hang out with them, joke with them, and have them bond with each other too freely around the classroom.

When the next day comes return back to the serious routines and productive energy first. Etc etc.