Ninh Binh or much better Trang An is the best place to start your adventures and explorations of the nearby limestone scenery as well called "Halong Bay on land". Trang An is part of the UNESCO World Heritage and quite a remarkable complex of a vast landscape. There is a lot of Ecologic Tourism going on in the area and people are friendly and easy to go.
There are a lot of different things in the area worth to visit and to experience for example the Trang An Grottos, the Mua Cave and last but not least the well known Grottos of Tam Coc. Pagodas and temples are plenty here to visit and to explore.
Here a short video from the trip from Hanoi to Trang An and some images from the area.