▲其实此处填不定式和动名词（不是现在分词）都可以。比如《朗文实用英语搭配词典》就有这样一组划等号的同义句：

It was fun to go on the roller coaster.

=It was fun going on the roller coaster. 

乘过山车很好玩。

▲不过，虽然 it is fun 后接不定式或动名词都可以，但若 fun 前有 no, much, great, a lot of 等修饰时，似乎后接动名词的用法比较普遍。如：

It’s no fun eating on your own. 一个人吃饭没意思。

It’s great fun sailing a boat. 扬帆驾舟十分有趣。

It can’t be much fun living next door to him. 与他为邻决没什么意思。 

如果 fun 前面没有修饰语，则后接不定式时比较普遍。如：

It would be fun to have a beach party. 在海滩举行聚会会很有意思。

It is more fun to go with someone than to go alone. 偕伴同去比自己独自去好玩。 

It’s fun to be cast as the villain for a change. 改变一个去扮演一个坏人也很有意思。

We thought it would be fun to take a ride in the boat. 我们觉得乘小船游玩会很有意思。