This is an important venue to the residents of Hanoi, but there's not a huge amount to see here. The original features were interesting, such as an enormous gnarled tree and some sunken enclosed pools. There's various large stone ornaments fashioned like turtles (but look like tombstones). Sadly none of the guide text is in English, so I can only guess what these were.
I strongly recommend you do not go during Tet (Chinese New Year). This is a traditional place of worship, and the place was rammed full to the point of it being annoying. We had a quick whizz round and decided to leave as there was little to see, and trying to navigate the massive crowds was not much fun