You are assuming that fun is a factor that would play a role for a deity, which I myself would kind of agree with since in most religious books it is often indicated that we are the image of god or if not share some similarities. I would put some meaning into that claim since there is the feeling which we have called , feeling like god, or playing god. A name we give if we create something ourselves or feel especially powerful. From that we can derive 2 things , first that God is a creator , second that he is powerful and third that he transmits some properties of his own to his creations.
So suppose one of these properties was that of having fun which we humans share , then God himself should have some resembling property or at least a borderline property which may be derived from other properties. So since God’s property of being able to have fun is established the question shifts to what god may enjoy.
Since the image property is established , saying all the things humans do would be too easy of an analogy ,so as factors I would also like to add :
1)being allpowerful ,
2)being able to create anything that a human can imagine and beyond ,
4)and finally having everlasting life.
So before adding these lastly added properties into consideration let’s consider what we as humanbeings would really enjoy if we created beings at our disposal. The first thing that comes into my mind is the scientific observation of bacteria or if not that other sentient beings , such as observed hatched miniscule beings as in the episode of the Simpsons if you have watched it(Season 8 Episode 1).
In that episode Lisa creates intelligent life by accident in a petri dish which goes through many phases of development from being underdeveloped cavemen to modern 21. century beings with developed ideals and technology in a matter of days.
Now to the root of the intriguing or "fun" part which makes the episode fun to watch. It is the not knowing, of how and where the development will go, or even if you may predict the outcome since it becomes later pretty clear after the first day in which it can be seen that the little beings in the dish invent fire, that the temporal development will be very similar to the historical human development we know, which ,in the next scene is revealed to be the case.
Now to the reason of this parallel.
What the episode plays on the most is the curiosity , of the creator who created and the audience who observed it all which would be us. We can observe and we can create, we can even intervene (there is a scene where Bart crushes a whole continent by putting his finger in the dish) if we do wish to do so and to the part of being everlasting I would draw from the parallel that Lisa as a human being lives a way longer life than anything that the beings in the dish could even imagine. Altogether I would put Lisa , the creator, Bart the intervener and us the audience , the watcher in one basket , calling it God. To be more abstract I would like to add , that the personalities of those three roles may vary and be interchangeable so not to say God has similar personality traits. It is enough although to say that he does fill the roles of those three and that his wishes and interventions are not arbitrary.
Now also I am aware that living indefinitely is a completely different matter than that of living a very long life but since being immortal raises again the question of whether enjoyment can be felt at all and other tough questions , I chose to oversimplify that fact, so to skip that question in a discrete matter.
Now since enjoyment or fun can be felt by a deity, the reason of why god would have any fun in creating us is that God created something in which he does not have the information of everything that will occur. If he did , then there would be no sense in creating a scenario in which he saw all the lives of everyone being played out in front of his eyes. So to assume that God knew all the rules and every detail of the playground with all its laws and functionalities and outcomes but not how the little sentients are going to play is what would explain the previously added premises.
So the ultimate answer would be fascination for the unknown.