I don't know your specific work/life situation, so I won't pretend to know what all the constraints are, but if you'll indulge me I want to try to challenge the idea that play is something you can only do away from all the dull stuff.

If you have a "bring your inner child to work day" would anyone notice? And what if it works out for you and you start to smuggle them in every day? It doesn't have to mean acting like a big ol' goofball in meetings, but it could be approaching the stuff you have to do from the perspective of openness, what-ifs and sometimes asking the kind of questions (like "why?") that adults usually don't because they fear it may mark them as someone who doesn't know everything and have all the answers. Or when it comes to tasks that genuinely have fewer creative opportunities, setting small work-related challenges for yourself, achieving high scores nobody else knows about, etc. (I feel like I read a blog post or something about this, but I can't remember what.) Or just having a secret laugh at what five-year-old you might have thought of a co-worker or some situation you observed. If the "adult mask" you mentioned is working for you, then keep wearing it. But you get to decide what goes on behind it.

I don't want to sound like an apologist for working all the time, and you should also be making time for actual play, on your own time, that uniquely benefits you! But I also don't want you buying into the notion that work time, where you spend so much of your life, should be reserved for suffering in a straight-jacket.