it's fun being taken to the zoo

it is fun to be taken to a zoo

给你些常用的例句吧

It's fun to be a volunteer

its fun being a girl

It's fun to take a trip

It's Fun To Be Fooled

'It's fun to be alive'

It's fun being married to him

fun当名词时，
当“享乐，乐趣，快乐，享乐的事”的意思讲时，
有be fun doing
eg: It's not much fun going to a party on your own.
(独自一人参加聚会没什么意思）
fun当形容词时，
当“逗乐的，有趣的，使人开心的”的意思讲时，
有be fun to do
eg: She's really fun to be with.
(和她在一起真开心）
参考资料：Oxford Advanced Learner's English-Chinese Dictionary

我觉得应该是to be....

it's fun being taken to the zoo
很简单，这个是现在进行时的被动语态。
it's fun to be taken to the zoo
这个则是一般将来时的被动语态了。

It's fun doing sth 干...是有趣的

Being taken to the zoo is fun.

形式主语句。