fun vs merriment

fun and merriment both are nouns.

Word	Noun	Adjective	Verb	Adverb
fun	Yes	No	No	No
merriment	Yes	No	No	No

As nouns, fun and merriment are synonyms defined as:

Other synonyms of fun include playfulness.

fun (noun)	merriment (noun)
activities that are enjoyable or amusing	activities that are enjoyable or amusing
violent and excited activity	a gay feeling
a disposition to find (or make) causes for amusement
verbal wit or mockery (often at another's expense but not to be taken seriously)