enormous, excellent (BrE), good, great, terrific, tremendous
pure, sheer
clean, good clean, harmless, innocent, plain (esp. AmE)
The boys' game started as harmless fun but ended in tragedy.
It was just plain fun.

family

be, become, get
look, seem, sound
have
We had a lot of fun at Mick's party.
He decided to have a little fun at his friend's expense.

ruin, spoil
We won't let the rain spoil our fun.

want
find

day
She organized an annual fun day for local children.

for fun
I write for fun, not because I expect to make money.

in fun
I only said it in fun—don't take it seriously!

fun for
a game that's fun for kids

be no fun
It's no fun getting up at 4 a.m. on a cold, rainy morning.

a bit of fun
I was only having a bit of fun.

full of fun
This movie is full of fun.

fun and games
School isn't all fun and games.

just for fun, just for the fun of it
They took up motor racing just for the fun of it.

miss the fun
You're missing all the fun!

the real fun
That's when the real fun started!

a sense of fun
You have to have a sense of fun to be a good teacher.

suck (all) the fun out of sth (AmE), take the fun away from sth, take the fun out of sth
Must you take all the fun out of everything?

Fun is used with these nouns:
day, diversion, evening, fair, fur, game, hobby, movie, night, part, read, ride, run, time