Why do you go there?
It's cheaper and funner, that's why.
That was the funnest!
The use of fun as an adjective is mainstream now, and -er and -est is becoming increasingly natural with time. It's a classic example of how small words evolve in English.
That said, I wouldn't say there is ever a need to use them. That would upset a few people that you don't need to.
In your example, in particular, "more and more fun" sounds smooth.