Learners of English often make sentences like this:
1) X I went to Disneyland and it was very fun.
Sentence (1) is incorrect. But we can say this:
2) I went to Disneyland and it was very enjoyable.
The point is that "fun" is an uncountable noun, as can be seen in these sentences:
3) I went to Disneyland and had a lot of fun.
4) I did not have much/any fun at Disneyland.
Naturally, this sentence is also correct English:
5) I went to Disneyland and it was great fun.
Sentence (5) is grammatically similar to the following sentences:
6) I felt great happiness/joy.
7) I felt deep love/pain.
However, in informal English "fun" can be used as an adjective. But only as an adjective which comes before a noun, as in the following:
a fun thing 楽しいこと／物
a fun time 楽しい時間
a fun day out 楽しい休日のおでかけ
a fun party 楽しいパーティ
a fun person 愉快な人
a fun film 楽しい映画