I visited the Trang An Landscape in April of 2015 and really enjoyed the experience. It is a beautiful area that you can spend quite a lot of time exploring.

In order to see it, I did the standard Trang An boat tour which costs 150,000 dong and takes about two hours. While it is quite popular with (local) tourists groups, you are in a boat of just six people so it is not as touristy as it could be. The boat takes you through caves and to little waterscapes you wouldn't be able to access otherwise.

There is no shade on the boat so, a word of warning, make sure to take sun protection and some water!

To get to Trang An you can get a direct train from Hanoi to Ninh Binh and then a taxi from there (about 75,000 dong).

I actually stayed overnight near Ninh Binh (you can get here on a direct train from Hanoi) and enjoyed spending a couple of days cycling around the region and seeing some other natural areas. They may not technically be part of the WHS, but they look the same.