Oxford dictionary states that "fun" is a noun, a verb and an adjective. It even shows the forms of the adjective as fun, funner and funnest, although I must admit that I have never heard of funner or funnest. When fun is used as an adjective it must be followed by a noun,
e.g., It was a fun activity
e.g., It was a/the more fun activity.
e.g., It was the most fun activity.
And if anyone believes the Oxford dictionary:
e.g., It was a fun activity
e.g., It was the funner activity.
e.g., It was the funnest activity.
Wow - those just don't sound right to me, but who am I to argue with Oxford ...