Kaijuu 8-gou: Hoshina no Kyuujitsu

I rated this episode a 7 though my actual enjoyment of it would be a 6.

It's a tonal shift from the normal series and as such, watching right after finishing the series, felt a little jarring to me. It's much more silly, laid back and "filler-ish" This is why my enjoyment was only a 6.
HOWEVER.
AS a Filler/side episode I think it actually did it's job at that specific role quite well. They are often a different tone and more laid back and "fluff" as it were. I think for the genre of "side-episode" where you're not going to the ... dramatic tension route, or the sterotypical beach/swims-suit episode, it was done well enough. A touch of the absurd, a reason why the main character wasn't present etc.

It's not a "have to watch" or even probably a "should watch" episode. It's a "watch if you feel like it" episode, and for that particular purpose is fine.

Have to say, either you are a filler person or you are not!
Some people despise fillers hence they might dread the classics, some people love them.
I care about every related piece of media released for that one title.
If you are craving for more Kaiju No. 8, or any other anime, then you should definitely watch the extra content to wash the craving away!
All that being said, this one is about how the Division Three Task Force is on its off day without the presence of the main character, going through their off day and coming back full circle, ending up working and helping people on ... their day off, an almost representative episode of the life work in Japan, yet not truly.
Something I have to point out that took out points for me was the fact that the animation in this felt completely dull and soulless unlike the main show!
Like it was done by a smaller sub-studio with less time and less resources.
There were no fights so the animation doesn't truly matter, but it does to me and it left a bad after taste.

Kaiju No. 8: Hoshina’s Day Off is a mostly unremarkable, slice-of-life and comedy-focused episode that is centered around the cadets of the Third Division of the Defense Force outside of their mission as they secretly follow their vice-captain. The plot structure is about as generic as it comes: A misunderstanding that develops into something far more innocent than once assumed. Despite the production quality and animation remaining equal to the main series, the content within leaves much to be desired. At most, we gain a very brief look into their normal lives, but are otherwise left with little substance or narrative reason for this addition.