...I feel like I'm the only one who doesn't understand the question? ^^; Like...it feels vague to the point of absurdity...
Is...is that not the definition of 'fun'?? Something you look forward to and feel excited about doing? Why would there be a 'kind' of person who experiences fun; isn't that the vast majority of people??? o_O When you never look forward to anything, we usually call that clinical depression...
The question comes off as "what is joy" and I'm like...ಠ_ಠ??? Are we answering a writing question or a cry for help...?
...I'm going to assume it's a writing question, and try to rephrase it a few ways:
"What is it that makes a person physically vibrant and excited about having things to do?"
^This is really just a matter of personality, cultural norms, and upbringing-- some people are more inclined to physical displays of emotion, and some cultures encourage/discourage this, along with the people in their immediate environment. A physically-expressive person who grew up around reserved people and a reserved person who grew up around physically-expressive people could be similar in some ways and wildly different in others; there are a lot of variables.
"What is it that makes a person vibrant and excited about having anything to do?"
So my answer here is almost the same as the previous, just with a bit more specificity-- that is to say, some people have certain inclinations and life experiences that make activity, just in general, very stimulating and enjoyable. Basically, some people just really like to do stuff; they'd much rather be active than inactive.
And for other people, it really depends on how they're feeling at that particular time-- for example, I went on a brief vacation earlier this year, and for weeks afterward I still wanted to be constantly going places and meeting people. ^^; The thought of sitting in my room and drawing/writing every day after work (i.e. what I'd always done for, like...my whole life) felt boring to me; it took me a long time to settle back down.
"What is it that makes a person literally vibrant and excited about having things to do?"
P.S. I feel I have to point out that not everyone expresses 'excitement' in the same way (and if this really is a writing question, this point is very important). "Vibrant" and "excited" are actually two different things, especially depending on the source of the excitement...being excited to see a new movie is different from being excited to kill a mortal enemy, which is different from being excited start a difficult project. And all of those kinds of excitement can look different depending on where you fall on the neurodiversity scale, or even the gender expression scale. Like I said, there are a lot of variables.