Cat Ba is the largest island in Ha Long Bay, it is about 350 km². It also belongs to the most visited islands in Ha Long Bay, mainly by Vietnamese citizens. Ha Long Bay is a very touristy place, but it need not be everywhere, and especially in the smaller islands in the bay. Cat Ba Island is best to travel by motorbike or bicycle. On the island is possible to take a boat trip to another islands with some of the local fishermen or travel agencies.
Cat Ba National Park on the island is a home by many interesting ecosystems (forests, swamps, mangroves and reefs). You can rent here also kayaks. Cat Ba Island can be reached from the island of Tuan Chau by boat (Ha Long Bay) or from the Hai Phong city by ferry (Ben Binh harbor). But there is also a direct bus from Hanoi to Cat Ba Island. The island can also be a part of several days of hiking trips as a stop in Ha Long Bay. Read more information about Ha Long Bay and Cat Ba Island.
Now you can also very easily travel by bus from Cat Ba Island to Ninh Binh.
Read this > How to get from Cat Ba Island to Ninh Binh.